(docs): add a types badge

- to indicate to those who may not see the languages initially
  - e.g. on mobile GitHub, on NPM, etc
- badge relies on NPM, so won't work properly until new version is
  published

README.md

@@ -9,6 +9,8 @@
 [![dm](https://img.shields.io/npm/dm/mst-persist.svg)](https://npmjs.org/package/mst-persist)
 [![dw](https://img.shields.io/npm/dw/mst-persist.svg)](https://npmjs.org/package/mst-persist)
 
+[![typings](https://img.shields.io/npm/types/mst-persist.svg)](https://github.com/agilgur5/mst-persist/blob/master/src/index.ts)
+
 [![NPM](https://nodei.co/npm/mst-persist.png?downloads=true&downloadRank=true&stars=true)](https://npmjs.org/package/mst-persist)
 
 Persist and hydrate [MobX-state-tree](https://github.com/mobxjs/mobx-state-tree) stores.