Documentation for contributors

**WIP Do no merge yet**

This should fix #36.

This is the first version of a CONTRIBUTING file, the target of this is to explain how to start contributing on Pacto and some proposed guidelines. Those guidelines are (in theory) rules that the current contributors agree.

Please fell free comment, add, remove text.

I still have some doubts before merging to master:
- Not sure if the problems with the Gemfile.lock are properly explained.
- The way the guide is written is too much informal (I believe).
- English might not be perfect, please review it.