add a link for the website github URL to contributing page

Before:
![ember js_-_contributing_to_ember js__repositories](https://f.cloud.github.com/assets/514063/1853574/5c95bcfa-770e-11e3-96b9-08acd7d7e95f.png)

After:
![ember js_-_contributing_to_ember js__repositories](https://f.cloud.github.com/assets/514063/1853611/d74e82b0-770e-11e3-82ca-890bb2146ea0.png)