added comment to message.dart

Author: gaaclarke

packages/pigeon/pigeons/message.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.
 
+// This file is an example pigeon file that is used in compilation, unit, mock
+// handler, and e2e tests.
+
 import 'package:pigeon/pigeon.dart';
 
 class SearchRequest {