Add example for mobx-state-tree

We could use [`connectReduxDevtools `](https://github.com/mobxjs/mobx-state-tree/tree/master/packages/mst-middlewares#connectreduxdevtools) middleware of MST on React Native with RNDebugger.

No file or test is named. Start with the linked MST connectReduxDevtools middleware documentation and inspect RNDebugger's existing documentation or examples. Done means a React Native example demonstrates using the middleware with RNDebugger.

索引モデルが issue の本文から書いたものです。