Add copyright header (#60)

diff --git a/example/main.dart b/example/main.dart
index 46ba94b..b9d5bed 100644
--- a/example/main.dart
+++ b/example/main.dart
@@ -1,3 +1,7 @@
+// Copyright (c) 2020, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
 import 'package:characters/characters.dart';
 
 // Small API examples. For full API docs see: