Add a CONTRIBUTING file to instruct people on issues and pull requests

I just came across that if you have a CONTRIBUTING._something_ file on your project, GitHub will display a notice above the input box. 

![contributing](https://cloud.githubusercontent.com/assets/1441704/2830898/82bc78e4-cfb3-11e3-8d09-69c43d71f9ae.png)

Having one will make some issues to never be submited (like PRs of new themes) and other issues to be more descriptive, therefore driving down the issues number and the time they remain open.

For now I propose having a brief description of what to do (e.g. search before posting) and what not to do (e.g. submit new themes, or duplicate pull requests), and a link to a section in the wiki that can be better modified because it doesn't need to be modified and then merged.