message: Document how to download and install Firebase C++ SDK.
  Needed for Zooshi builds on Android.
diff: @@ -20,10 +20,14 @@ Building for Android {#zooshi_guide_building_android}
  - Install the `Google Play services` and `Android Support Library` packages
  from the [SDK Manager][].
  - Download the [Google Play Games C++ SDK][] and unpack into Zooshi's
  - `dependencies/gpg-cpp-sdk` directory.
  + `dependencies/gpg-cpp-sdk` directory, or set environment variable `GPG_SDK`
  + to the install directory.
  - For example, if you've fetched Zooshi to `~/zooshi/`, unpack
  the downloaded `gpg-cpp-sdk.v2.0.zip` to
  `~/zooshi/dependencies/gpg-cpp-sdk`.
  +- Download the [Firebase C++ SDK] and unpack into Zooshi's
  + `dependencies/firebase_cpp_sdk` directory, or set environment variable
  + `FIREBASE_SDK` to the install directory.
  # Building
  @@ -70,4 +74,5 @@ If only one device is attached to a workstation, the `-d` argument
  [WebP Precompiled Utilities]: https://developers.google.com/speed/webp/docs/precompiled
  [Windows prerequisites]: @ref building_windows_prerequisites
  [Google Play Games C++ SDK]: http://developers.google.com/games/services/downloads/
  + [Firebase C++ SDK]: https://firebase.google.com/docs/cpp/setup
  [Zooshi]: @ref zooshi_guide_overview
  