Create CONTRIBUTING.md

This is a copy of ansibles CONTRIBUTING.md adapted to ODOO, it is quite clear and friendly and helps get your way arround. As my knowledge is limited, I could not redact every secction. Thos secction who aren't yet redacted are idented as `code blocks` for convenience to the revisor.

[This Issue](https://github.com/odoo/odoo/issues/61) will be solved by this.