Add information about branches to contributing.md

During the bug squad session at JAB14 there was some confusion about which branch to use.

I think we can add this information to the contributing.md file. A link to this file ("guidelines for contributing") is shown whenever someones wants to open a Pull Request. See this screenshot:
![guidelines](https://cloud.githubusercontent.com/assets/1018684/3171727/a81e48aa-ebca-11e3-8f36-62fd7d54bf7a.png)

I'm sure this can be improved with other information as well.