Add badge for http://makeapullrequest.com/

Author: ValentinH

README.md

@@ -9,6 +9,7 @@ Status:
 
 Links:
 [![Join the chat at https://gitter.im/rzajac/angularjs-slider](https://img.shields.io/badge/GITTER-join%20chat-1dce73.svg?style=flat-square)](https://gitter.im/rzajac/angularjs-slider?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge&utm_content=badge)
+[![PRs Welcome](https://img.shields.io/badge/PRs-welcome-brightgreen.svg?style=flat-square)](http://makeapullrequest.com)
 
 Slider directive implementation for AngularJS, without any dependencies: [http://angular-slider.github.io/angularjs-slider](http://angular-slider.github.io/angularjs-slider/index.html).