Fix incomplete README for FirebaseAuth (#694)

Author: Paolo Rotolo

packages/firebase_auth/README.md

@@ -20,7 +20,7 @@ To use the firebase_auth plugin, follow the [plugin installation instructions](h
 Enable the Google services by configuring the Gradle scripts as such.
 
 1. Add the classpath to the `[project]/android/build.gradle` file.
-```
+```gradle
 dependencies {
   // Example existing classpath
   classpath 'com.android.tools.build:gradle:3.1.2'
@@ -30,7 +30,7 @@ dependencies {
 ```
 
 2. Add the apply plugin to the `[project]/android/app/build.gradle` file.
-```
+```gradle
 // ADD THIS AT THE BOTTOM
 apply plugin: 'com.google.gms.google-services'
 ```
@@ -48,17 +48,18 @@ Otherwise you will not be able to authenticate.
 ### Use the plugin
 
 Add the following imports to your Dart code:
-```
+```dart
 import 'package:firebase_auth/firebase_auth.dart';
 ```
 
-Initialize `FirebaseAuth`:
-```
+Initialize `GoogleSignIn` and `FirebaseAuth`:
+```dart
+final GoogleSignIn _googleSignIn = GoogleSignIn();
 final FirebaseAuth _auth = FirebaseAuth.instance;
 ```
 
 You can now use the Firebase `_auth` to authenticate in your Dart code, e.g.
-```
+```dart
 Future<FirebaseUser> _handleSignIn() async {
   GoogleSignInAccount googleUser = await _googleSignIn.signIn();
   GoogleSignInAuthentication googleAuth = await googleUser.authentication;
@@ -73,7 +74,7 @@ Future<FirebaseUser> _handleSignIn() async {
 
 Then from the sign in button onPress, call the `_handleSignIn` method using a future
 callback for both the `FirebaseUser` and possible exception.
-```
+```dart
 _handleSignIn()
     .then((FirebaseUser user) => print(user))
     .catchError((e) => print(e));