[Android WebAPKs] Add README with WebAPK newbie guide

BUG=None

Change-Id: I9452e29c5e9c7a70a5e64cd8cbfecffa3c834bf3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2364074Reviewed-by: Glenn Hartmann <hartmanng@chromium.org>
Commit-Queue: Peter Kotwicz <pkotwicz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#799642}

chrome/android/webapk/README

-WebAPKs are a proposal to improve homescreen shortcuts by:
-- Giving them a spot in the app launcher
-- Enabling web pages to open a WebAPK when a user taps on a URL which belongs to
-    a WebAPK
-- Improving resource and permission attribution
-
-In short, they attempt to providing additional glue to give Progress Web Apps
-deeper integration into the Android OS, similar to native apps.
-
-The directory is broken down as follows:
-- shell_apk/ - a template APK which can be customized for a given domain based
-    on its Web App Manifest: https://www.w3.org/TR/appmanifest/
-- libs/client/ - a client library meant for a host browser to interact with
-    WebAPKs
-- libs/common/ - common library used by the WebAPK and host browser
-- libs/runtime_library/ - a library which can be dynamically loaded by a
-    ShellAPK. A host browser provides the library which the ShellAPK uses to
-    implement the majority of its functionaly. For chrome, this library is
-    packaged into Chrome's assets file and extracted as needed. This approach
-    ensures that the majority of the WebAPK logic is shared and can be updated
-    as often as the host browser, without the need for updating each individual
-    WebAPK.

chrome/android/webapk/README.md

+WebAPKs
+--
+A WebAPK is an APK which is installed by "Add to Home screen" in the Chrome app menu
+provided that the website meets the
+[requirements](https://web.dev/install-criteria/).
+
+Sample site which meets WebAPK requirements
+--
+[https://pwa-directory.appspot.com/](https://pwa-directory.appspot.com/)
+
+Code layout
+--
+- shell_apk/ - Code for APK which is generated by the Chrome server. Not compiled
+into Chrome.  
+- libs/client/ - Library which is compiled into Chrome. Useful if another browser
+implemented WebAPKs.  
+- libs/common/ - Library with code which is used by both WebAPK shell and Chrome. WebAPK shell
+and Chrome might use different versions of the library.
+- libs/runtime_library/ - Library which is stored in the Chrome APK's assets and which is extracted
+from the Chrome APK by the WebAPK at runtime. This approach ensures
+that the majority of the WebAPK logic is shared and can be updated as often as
+CHrome, without the need for updating each individual WebAPK.
+
+Installing WebAPK from Chrome Developer Build
+--
+To enable a developer build of Chrome to install WebAPKs run the following
+commands:
+
+`adb root`  
+`adb shell am broadcast -a com.google.gservices.intent.action.GSERVICES_OVERRIDE -e finsky.webapk.do_webapk_install_package_check false`  
+`adb shell am broadcast -a com.google.gservices.intent.action.GSERVICES_OVERRIDE -e finsky.webapk.do_webapk_install_signing_check false`
+
+Building WebAPK shell locally
+--
+It is possible to build a test WebAPK and bypass the generation on the WebAPK
+server.
+
+On Android, build  
+//chrome/android/webapk/shell_apk:new_splash_webapk
+(new_splash='Android')  
+On ChromeOS, build  
+//chrome/android/webapk/shell_apk:webapk
+
+Both can be customized via [shell_apk/manifest/bound_manifest_config.json](https://source.chromium.org/chromium/chromium/src/+/master:chrome/android/webapk/shell_apk/manifest/bound_manifest_config.json)
+
+To make a locally built WebAPK launch Chrome in 'WebAPK mode':
+1) Set the --skip-webapk-verification Chrome command line flag
+2) Ensure that the 'scope_url_\*' parameters in bound_manifest_config.json
+   match a directory which contains the 'start_url' parameter. In 99% of
+   test cases the other parameters can use arbitrary URL origins.
+
+   Example:
+   ```
+   "intent_filters": {
+     "scope_url_scheme": "https",
+     "scope_url_host": "foo.com",
+     "scope_url_path_type": "android:pathPrefix",
+     "scope_url_path": "/bar/"
+   },
+   "start_url": "https://foo.com/bar/baz/start.html",
+   ```