Example: Horizon and React Native

[mglukhovsky]

We have a lot of users who are experimenting with Horizon and React Native (e.g. rethinkdb/horizon#255 and rethinkdb/horizon#477), and an open issue for building an example app: rethinkdb/horizon#285

We should have an article that introduces the example and describes using Horizon with React Native.

[lirbank]

Relates to rethinkdb/horizon#750