added download badge

README.md

@@ -7,6 +7,8 @@ _Simple, scalable state management_
 [![Coverage Status](https://coveralls.io/repos/mobxjs/mobx/badge.svg?branch=master&service=github)](https://coveralls.io/github/mobxjs/mobx?branch=master)
 [![Join the chat at https://gitter.im/mobxjs/mobx](https://badges.gitter.im/Join%20Chat.svg)](https://gitter.im/mobxjs/mobx?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge&utm_content=badge)
 
+![npm install mobx](https://nodei.co/npm/mobx.png?downloadRank=true&downloads=true)
+
 * Installation: `npm install mobx --save`. React bindings: `npm install mobx-react --save`. To enable ESNext decorators (optional), see below.
 * [Ten minute, interactive MobX + React tutorial](https://mobxjs.github.io/mobx/getting-started.html)
 * [Official documentation and API overview](https://mobxjs.github.io/mobx/refguide/api.html)