Update contributing section using Github Guideline.

Github added support for sharing the preferred policy for contributions with the folks wanting to collaborate with you on the project.   

[https://github.com/blog/1184-contributing-guidelines](https://github.com/blog/1184-contributing-guidelines)

![68747470733a2f2f6769746875622d696d616765732e73332e616d617a6f6e6177732e636f6d2f736b697463682f6973737565732d32303132303931332d3136323533392e6a7067](https://cloud.githubusercontent.com/assets/301291/3485020/ec7214f2-03bf-11e4-91dd-1eac8afc6a23.jpeg)