Add license badge (#2446)

MIT License badge added in README.md file

README.md

@@ -7,6 +7,7 @@
 [![npm downloads](https://img.shields.io/npm/dm/axios.svg?style=flat-square)](http://npm-stat.com/charts.html?package=axios)
 [![gitter chat](https://img.shields.io/gitter/room/mzabriskie/axios.svg?style=flat-square)](https://gitter.im/mzabriskie/axios)
 [![code helpers](https://www.codetriage.com/axios/axios/badges/users.svg)](https://www.codetriage.com/axios/axios)
+![license: MIT](https://img.shields.io/badge/License-MIT-blue.svg)
 
 Promise based HTTP client for the browser and node.js