Add a README.md file to [...]/browser/settings/website

As we expect new people to join the permissions team, I've written
a short overview of the site settings screens in Android to help people
start up when working with these.

Change-Id: I176a5cf7a8ff29dc1b28b8c3f82062bb0e5db8f5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2077437
Commit-Queue: Andy Paicu <andypaicu@chromium.org>
Reviewed-by: Kamila Hasanbega <hkamila@chromium.org>
Cr-Commit-Position: refs/heads/master@{#745931}

chrome/android/java/src/org/chromium/chrome/browser/settings/website/README.md

+# Content Settings and Site Settings in Android
+
+[TOC]
+
+## Overview
+
+There are 3 main user-facing screens when it comes to displaying content
+settings status to the user:
+
+*   [Site Settings](#Site-Settings) provides a link to `All Sites` and an
+    overview of all content settings. Supported by the
+    [SiteSettings](https://cs.chromium.org/chromium/src/chrome/android/java/src/org/chromium/chrome/browser/settings/website/SiteSettings.java?g=0)
+    class.
+*   [Settings Category](#Settings-Category) displays a specific content setting
+    in detail. This usually includes a toggle to enable/disable the content
+    setting browser-wide, any category-specific options, and lists of sites that
+    are `allowed`/`blocked`/`managed`. The same screen is used to display the
+    `All Sites` category which displays all sites that have any content settings
+    set. Supported by the
+    [SingleCategorySettings](https://cs.chromium.org/chromium/src/chrome/android/java/src/org/chromium/chrome/browser/settings/website/SingleCategorySettings.java?type=cs&g=0)
+    class.
+*   [Website Settings](#Website-Settings) displays site-specific content
+    settings information. It displays a list of content settings that the site
+    is either allowed or blocked from and a button to reset all content settings
+    state for this site. Supported by the
+    [SingleWebsiteSettings](https://cs.chromium.org/chromium/src/chrome/android/java/src/org/chromium/chrome/browser/settings/website/SingleWebsiteSettings.java?type=cs&g=0)
+    class.
+
+All of these implement `PreferenceFragmentCompat` and use a layout xml to define
+the preferences that form the screens.
+
+## Site Settings
+
+This screen is relatively static in nature, being a central hub for settings
+categories.
+
+[site_settings_preferences.xml](https://cs.chromium.org/chromium/src/chrome/android/java/res/xml/site_settings_preferences.xml?q=site_settings_preferences&sq=package:chromium&g=0&l=1)
+lists all preferences.
+
+[configurePreferences()](https://cs.chromium.org/chromium/src/chrome/android/java/src/org/chromium/chrome/browser/settings/website/SiteSettings.java?type=cs&g=0&l=53)
+will remove some preferences based on command line switches and feature
+availability.
+
+[updatePreferenceStates()](https://cs.chromium.org/chromium/src/chrome/android/java/src/org/chromium/chrome/browser/settings/website/SiteSettings.java?type=cs&g=0&l=72)
+goes through all preferences and configures cool things like: title, summary,
+icon, and click listeners.
+
+## Settings Category
+
+This screen contains the logic used to display specific categories, as well as
+the 'All sites' category."
+
+[website_preferences.xml](https://cs.chromium.org/chromium/src/chrome/android/java/res/xml/website_preferences.xml?q=website_preferences&sq=package:chromium&g=0&l=1)
+outlines the basic layout of the screen displaying the global settings for a
+category.
+
+The
+[mCategory](https://cs.chromium.org/chromium/src/chrome/android/java/src/org/chromium/chrome/browser/settings/website/SingleCategorySettings.java?type=cs&g=0)
+member holds which category needs to be displayed and is queried to dictate the
+customization of the page.
+
+[configureGlobalToggles()](https://cs.chromium.org/chromium/src/chrome/android/java/src/org/chromium/chrome/browser/settings/website/SingleCategorySettings.java?type=cs&g=0&l=821)
+handles most of the customization of the screen. It will go through all
+preferences and set their values, their visibility status as well as set up
+preference change listeners.
+
+[getInfoForOrigins()](https://cs.chromium.org/chromium/src/chrome/android/java/src/org/chromium/chrome/browser/settings/website/SingleCategorySettings.java?type=cs&q=configureGlobalToggles&g=0&l=143)
+is used to fetch websites information via a
+[WebsitePermissionsFetcher](https://cs.chromium.org/chromium/src/chrome/android/java/src/org/chromium/chrome/browser/settings/website/WebsitePermissionsFetcher.java).
+When the data is available it rebuilds the screen and adds the websites to the
+`allowed`/`blocked`/`managed` lists.
+
+[onPreferenceChange()](https://cs.chromium.org/chromium/src/chrome/android/java/src/org/chromium/chrome/browser/settings/website/SingleCategorySettings.java?type=cs&g=0&l=464)
+is the main listener of user input. When the user updates a preference, it will
+identify the changed preference, use bridges to call the native code that
+handles updating user content settings
+([WebsitePreferenceBridge](https://cs.chromium.org/chromium/src/chrome/android/java/src/org/chromium/chrome/browser/settings/website/WebsitePreferenceBridge.java)
+and
+[PrefServiceBridge](https://cs.chromium.org/chromium/src/chrome/browser/preferences/android/java/src/org/chromium/chrome/browser/preferences/PrefServiceBridge.java?q=PrefServiceBridge&sq=package:chromium&g=0&l=18))
+and then update the screen to ensure the new preference value is taken into
+account including whatever ramifications that might have.
+
+The `All Sites` list and the three `allowed`/`blocked`/`managed` lists are
+populated with the custom
+[WebsitePreference](https://cs.chromium.org/chromium/src/chrome/android/java/src/org/chromium/chrome/browser/settings/website/WebsitePreference.java)
+type of preference which allows clicks to navigate to the
+[SingleWebsiteSettings](https://cs.chromium.org/chromium/src/chrome/android/java/src/org/chromium/chrome/browser/settings/website/SingleWebsiteSettings.java)
+fragment.
+
+## Website Settings
+
+This screen is used to display the relevant information for a specific website.
+
+[single_website_preferences.xml](https://cs.chromium.org/chromium/src/chrome/android/java/res/xml/single_website_preferences.xml?q=single_website_preferences&sq=package:chromium&dr)
+outlines the layout of the screen.
+
+The underlying model object is a
+[Website](https://cs.chromium.org/chromium/src/chrome/android/java/src/org/chromium/chrome/browser/settings/website/Website.java)
+object: the
+[mSite](https://cs.chromium.org/chromium/src/chrome/android/java/src/org/chromium/chrome/browser/settings/website/SingleWebsiteSettings.java?l=108)
+member. This is either directly provided by the activity initiator, or
+alternately fetched via a
+[SingleWebsitePermissionsPopulator](https://cs.chromium.org/chromium/src/chrome/android/java/src/org/chromium/chrome/browser/settings/website/SingleWebsiteSettings.java?l=121)
+based on the website address.
+
+[displaySitePermissions()](https://cs.chromium.org/chromium/src/chrome/android/java/src/org/chromium/chrome/browser/settings/website/SingleWebsiteSettings.java?l=300)
+is used to configure the screen and populate it with data from
+[mSite](https://cs.chromium.org/chromium/src/chrome/android/java/src/org/chromium/chrome/browser/settings/website/SingleWebsiteSettings.java?l=108),
+mostly by calling
+[setUpPreference()](https://cs.chromium.org/chromium/src/chrome/android/java/src/org/chromium/chrome/browser/settings/website/SingleWebsiteSettings.java?g=0&l=332)
+on every preference as well as specific custom setup functions.
+
+[onPreferenceChange()](https://cs.chromium.org/chromium/src/chrome/android/java/src/org/chromium/chrome/browser/settings/website/SingleWebsiteSettings.java?g=0&l=879)
+and
+[onPreferenceClick()](https://cs.chromium.org/chromium/src/chrome/android/java/src/org/chromium/chrome/browser/settings/website/SingleWebsiteSettings.java?g=0&l=896)
+are responsbile for listening to user input and updating the content settings
+accordingly.