Fix url of "Contributor's Guide"

The original url link to rtfd.org which ends up in the following page:

![screen shot 2013-07-12 at 5 29 27 pm](https://f.cloud.github.com/assets/116473/787271/a413bc82-ead5-11e2-9cf3-97118091bd5a.png)

Changing rftd to readthedocs fix the problem.