Adding JSON example. (#1)

jsonexample/README.md

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+# jsonexample
+
+A Flutter sample app that deserializes a set of JSON strings using three
+different libraries: `dart:convert`, `json_serializable`, and
+`built_value`.
+
+## Goals for this sample
+
+* Help you decide which of the three most common libraries for
+  deserializing JSON is right for your project.
+* Provide you with example code for deserializing:
+  * Simple and nested objects
+  * Lists of primitive values
+  * Maps containing primitive values
+
+## The important bits
+
+### `json_strings.dart`
+
+The actual JSON to be deserialized.
+
+### `dart_convert`/`json_serializable`/`dart_convert`
+
+These folders contain code used to deserialize the simple and complex
+object models using one of the three libraries. They do the work of
+instantiating models and populating their fields.
+
+### `tab_pages.dart`
+
+Each tab page in the app deserializes one type of data with one library.
+These Widgets show what app code using the above libraries might look like.
+
+## Questions/issues
+
+If you have a general question about JSON serialization in Flutter, the
+best places to go are:
+
+* [The FlutterDev Google Group](https://groups.google.com/forum/#!forum/flutter-dev)
+* [The Flutter Gitter channel](https://gitter.im/flutter/flutter)
+* [StackOverflow](https://stackoverflow.com/questions/tagged/flutter)
+
+If you run into an issue with the sample itself, please file an issue
+in the [main Flutter repo](https://github.com/flutter/flutter/issues).
+