Merge pull request #2634 from jbampton/add-license-button

Add MIT License button from shields.io

Author: etpinard

README.md

@@ -2,6 +2,7 @@
 
 [![npm version](https://badge.fury.io/js/plotly.js.svg)](https://badge.fury.io/js/plotly.js)
 [![circle ci](https://circleci.com/gh/plotly/plotly.js.png?&style=shield&circle-token=1f42a03b242bd969756fc3e53ede204af9b507c0)](https://circleci.com/gh/plotly/plotly.js)
+[![MIT License](https://img.shields.io/badge/License-MIT-brightgreen.svg)](https://github.com/plotly/plotly.js/blob/master/LICENSE)
 
 Built on top of [d3.js](http://d3js.org/) and [stack.gl](http://stack.gl/),
 plotly.js is a high-level, declarative charting library. plotly.js ships with over 20