Added a readme for the GSI C++ Library.

Change-Id: I3a4f6b099a9de7316dd316c6f3fa89e5bd99353a

Author: johnb003

staging/native/README.md

@@ -1,13 +1,55 @@
-TODO(butterfield): This is a placeholder to satisfy the Cocoapods library podspec.
+# Google Sign-In C++ SDK
+_Copyright (c) 2018 Google Inc. All rights reserved._
 
-To validate the podspec run:
 
-pod lib lint GoogleSignInCpp.podspec
+## Overview
 
-To build the testapp:
+This is the Google Sign-In C++ API, for Android and iOS. The intent is to unify
+the iOS (Objective-C) and Android (Java) native interfaces into a single common
+C++ interface that abstracts all of the platform specific details, however
+currently only the Android implementation is supported.
 
-cd testapp
-pod install
-open testapp.xcworkspace
+This is intended for projects that require OAuth ID tokens for server auth
+codes.
+
+### Getting Started
+
+See
+[Google Sign-In for Android](https://developers.google.com/identity/sign-in/android/start)
+for information on getting started.
+
+### Dependencies
+
+On Android you'll need to add the dependency
+`com.google.android.gms:play-services-auth`, based on the
+[setup guide](https://developers.google.com/identity/sign-in/android/start-integrating).
+
+
+## Configuring the application on the API Console
+
+To authenticate you need to create credentials on the
+API console for your application. The steps to do this are available on
+[Google Sign-In for Android](https://developers.google.com/identity/sign-in/android/start).
+In order to access ID tokens or server auth codes, you also need to configure a
+*web* client ID. From the
+[Credentials Page on the API Console](https://console.developers.google.com/apis/credentials)
+select the "Create credentials" dropdown, and choose OAuth Client ID. On the
+next page choose Web Application, name the client whatever you like, and then
+click "Create".
 
-(build and run)
+
+## How to build the sample
+
+First be sure to edit the sample source configuration:
+
+In testapp/src/common_main.cpp, replace "YOUR_WEB_CLIENT_ID_HERE" with your web
+client id, from the configuration step above.
+
+### Building for Android
+
+The android build is setup using gradle.
+
+Navigate to the testapp directory, and build using the "build" target:
+
+cd testapp
+./gradlew build