a welcome message in gallery (#7048)

because this is the most full-featured sample we have.

examples/flutter_gallery/lib/main.dart

@@ -2,6 +2,9 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+// Thanks for checking out Flutter!
+// Like what you see? Tweet us @flutterio
+
 import 'package:flutter/widgets.dart';
 
 import 'gallery/app.dart';