[Android][Settings] Add README

Change-Id: I2368bbe4f12c3ea9c7fef6071f58c8db33a1f9fa
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2068883Reviewed-by: Sky Malice <skym@chromium.org>
Commit-Queue: Natalie Chouinard <chouinard@chromium.org>
Cr-Commit-Position: refs/heads/master@{#743741}

chrome/browser/settings/README.md

+# Chrome for Android Settings
+
+## Getting Started
+
+The Android developer [Settings
+guide](https://developer.android.com/guide/topics/ui/settings) is the best place
+to start before contributing to Chrome for Android's settings.
+
+## Helper Classes
+
+Many common utility functions that are useful for developing settings screens in
+Chrome for Android can be found in `//chrome/browser/settings/android`.
+
+## Widgets
+
+The `widget` subdirectory contains a number of extensions of AndroidX
+[Preference](https://developer.android.com/reference/androidx/preference/Preference)
+classes that provide Chrome-specific behavior (like Managed preferences) or
+common Chrome UI components (like buttons).
+
+The base Preference classes included in the AndroidX Preference library can also
+be used directly in Chrome for Android Settings screens.