Adding JSON example. (#1)

jsonexample/lib/built_value/built_value_serializers.dart

@@ -0,0 +1,25 @@
+// Copyright 2018 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+library serializers;
+
+import 'package:built_collection/built_collection.dart';
+import 'package:built_value/serializer.dart';
+import 'package:built_value/standard_json_plugin.dart';
+import 'package:jsonexample/built_value/built_complex_object.dart';
+import 'package:jsonexample/built_value/built_simple_object.dart';
+
+part 'built_value_serializers.g.dart';
+
+@SerializersFor(const [
+  BuiltSimpleObject,
+  BuiltComplexObject,
+])
+
+// By default, `built_value` serialization uses lists and is not compatible
+// with other JSON formats. If you'd like to serialize data using a map-based
+// JSON approach (which is what you'll find in the json_strings.dart file in
+// this project), you can add the StandardJsonPlugin as you see here.
+final Serializers serializers =
+    (_$serializers.toBuilder()..addPlugin(StandardJsonPlugin())).build();