add prettier badge

Author: tiagoschenkel

README.md

@@ -7,5 +7,6 @@ RobotlegsJS SignalCommandMap Extension
 [![Test Coverage](https://codeclimate.com/github/RobotlegsJS/RobotlegsJS-SignalCommandMap/badges/coverage.svg)](https://codeclimate.com/github/RobotlegsJS/RobotlegsJS-SignalCommandMap/coverage)
 [![npm version](https://badge.fury.io/js/%40robotlegsjs%2Fsignalcommandmap.svg)](https://badge.fury.io/js/%40robotlegsjs%2Fsignalcommandmap)
 [![Greenkeeper badge](https://badges.greenkeeper.io/RobotlegsJS/RobotlegsJS-SignalCommandMap.svg)](https://greenkeeper.io/)
+[![styled with prettier](https://img.shields.io/badge/styled_with-prettier-ff69b4.svg)](https://github.com/prettier/prettier)
 
 A port of Robotlegs Extensions SignalCommandMap to TypeScript.