[gmscore] Add Google Play services documentation

docs/google_play_services.md

Index: docs/google_play_services.md
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+# Google Play services in Chrome for Android
+
+[TOC]
+
+Google employee? See [go/chrome-gms](https://goto.google.com/chrome-gms) for
+more info.
+
+## General setup
+
+The Google Play services are a combination of [services][play_store] exposed on
+Android devices and [libraries][dev_doc] to interact with them. Chrome relies
+on them for critical features like Sign in, Feedback or Cast.
+
+The standard way of adding the Google Play services as a dependency to a project
+is to import it through the Android SDK manager as a Maven repository. That
+repository contains multiple versions of the library split into separate APIs
+(for example Cast, GCM, Android Pay, etc). To avoid downloading a lot of data we
+don't need to build Chrome, android checkouts of Chromium download an archive
+containing only the APIs we currently need in Chrome, and in a single version.
+
+The up to date list of clients and version used can be seen in
+[//build/android/play_services/config.json][config_json_rel_path].
+
+**Note**: If you are working on a feature that requires different or more recent
+APIs, you will need to locally download the Google Play services SDK repository.
+
+The simplest way to download the latest SDK is to run:
+
+```
+$CHROMIUM_SRC/build/android/play_services/update.py sdk
+```
+
+Check out the help of that script for more info.
+
+[play_store]: https://play.google.com/store/apps/details?id=com.google.android.gms
+[dev_doc]: https://developers.google.com/android/guides/overview
+[config_json_rel_path]: ../build/android/play_services/config.json
+
+## Adding a dependency on new APIs
+
+As explained above, the default checkout has access to only a specific set of
+APIs during builds. If your CL depends on some APIs that are not included in the
+build, you will need [file an issue][bug_link] to request an update of our
+dependencies.
+
+Not doing so could make the CL fail on the trybots and commit queue. Even if it
+passes, it might fail on the internal bots and result in the CL getting
+reverted, so please make sure the APIs are available to the bots before
+submitting.
+
+[bug_link]:https://bugs.chromium.org/p/chromium/issues/entry?labels=Restrict-View-Google,pri-1,Hotlist-GooglePlayServices&owner=dgn@chromium.org&os=Android