add prettier badge

Author: tiagoschenkel

README.md

@@ -7,6 +7,7 @@ SignalsJS
 [![Test Coverage](https://codeclimate.com/github/RobotlegsJS/SignalsJS/badges/coverage.svg)](https://codeclimate.com/github/RobotlegsJS/SignalsJS/coverage)
 [![npm version](https://badge.fury.io/js/%40robotlegsjs%2Fsignals.svg)](https://badge.fury.io/js/%40robotlegsjs%2Fsignals)
 [![Greenkeeper badge](https://badges.greenkeeper.io/RobotlegsJS/SignalsJS.svg)](https://greenkeeper.io/)
+[![styled with prettier](https://img.shields.io/badge/styled_with-prettier-ff69b4.svg)](https://github.com/prettier/prettier)
 
 TypeScript port of [AS3 Signals](https://github.com/robertpenner/as3-signals).
 [13kb compressed](dist/signals.min.js).