docs(CONTRIBUTING): s/CONTRIBUTE.md/CONTRIBUTING.md/

To add a link on new PR page,
this file should be named as `CONTRIBUTING.md`.
* https://github.com/blog/1184-contributing-guidelines

Currently, the link to contributing is missing :<
![2014-07-01 22 33 38](https://cloud.githubusercontent.com/assets/290782/3443574/89f4ed2e-0124-11e4-9dbc-e4dd75bd9929.png)