Added documentation to the google sign in provider

lib/src/resources/google_sign_in_provider.dart

@@ -4,17 +4,30 @@ import 'package:firebase_auth/firebase_auth.dart';
 import 'package:google_sign_in/google_sign_in.dart';
 import 'package:rxdart/rxdart.dart';
 
+/// A Google authentication provider.
+///
+/// Connects to both Google and Firebase to authenticate a user.
 class GoogleSignInProvider {
   final GoogleSignIn _googleSignIn;
   final FirebaseAuth _auth;
 
+  // Instances of [GoogleSignIn] and [FirebaseAuth] can be injected for testing
+  // purposes. Don't remove.
   GoogleSignInProvider([GoogleSignIn googleSignIn, FirebaseAuth firebaseAuth])
       : _googleSignIn = googleSignIn ?? GoogleSignIn(),
         _auth = firebaseAuth ?? FirebaseAuth.instance;
 
+  /// A stream of [FirebaseUser] that notifies when a user signs in and out.
   Observable<FirebaseUser> get onAuthStateChange =>
       Observable(_auth.onAuthStateChanged);
 
+  /// Returns a future of [FirebaseUser].
+  ///
+  /// Initiates the Google authentication flow. Returns null if the flow fails
+  /// or gets cancelled.
+  ///
+  /// If the Google account is being used for the first time, a Firebase user
+  /// is also created thus this method also works as sign up.
   Future<FirebaseUser> signIn() async {
     try {
       final GoogleSignInAccount googleUser = await _googleSignIn.signIn();
@@ -34,10 +47,14 @@ class GoogleSignInProvider {
     return null;
   }
 
+  /// Returns a [FirebaseUser] if already signed in, returns null otherwhise.
   Future<FirebaseUser> getCurrentUser() async {
     return await _auth.currentUser();
   }
 
+  /// Signs a user out.
+  ///
+  /// Deletes the firebase user from disk and disconnects the Google user too.
   Future<void> signOut() async {
     await _googleSignIn.disconnect();
     await _auth.signOut();