docs(firebase_ui_auth): Handle UserCreated event in README example (#422)

Author: Rexios80

packages/firebase_ui_auth/README.md

@@ -44,15 +44,23 @@ class MyApp extends StatelessWidget {
   Widget build(BuildContext context) {
     final providers = [EmailAuthProvider()];
 
+    void onSignedIn() {
+      Navigator.pushReplacementNamed(context, '/profile');
+    }
+
     return MaterialApp(
       initialRoute: FirebaseAuth.instance.currentUser == null ? '/sign-in' : '/profile',
       routes: {
         '/sign-in': (context) {
           return SignInScreen(
             providers: providers,
             actions: [
+              AuthStateChangeAction<UserCreated>((context, state) {
+                // Put any new user logic here
+                onSignedIn();
+              }),
               AuthStateChangeAction<SignedIn>((context, state) {
-                Navigator.pushReplacementNamed(context, '/profile');
+                onSignedIn();
               }),
             ],
           );