README: Add MIT License badge

[ci skip]

Author: cvrebert

README.md

@@ -3,6 +3,7 @@
 [![NPM version](https://badge.fury.io/js/grunt-css-flip.svg)](http://badge.fury.io/js/grunt-css-flip)
 [![Build Status](https://travis-ci.org/twbs/grunt-css-flip.svg?branch=master)](https://travis-ci.org/twbs/grunt-css-flip)
 ![Development Status :: 5 - Production/Stable](https://img.shields.io/badge/maturity-stable-green.svg "Development Status :: 5 - Production/Stable")
+[![MIT License](https://img.shields.io/npm/l/grunt-css-flip.svg)](https://github.com/twbs/grunt-css-flip/blob/master/LICENSE)
 [![Dependency Status](https://david-dm.org/twbs/grunt-css-flip.svg)](https://david-dm.org/twbs/grunt-css-flip)
 [![devDependency Status](https://david-dm.org/twbs/grunt-css-flip/dev-status.svg)](https://david-dm.org/twbs/grunt-css-flip#info=devDependencies)