Add a CONTRIBUTING.md

[bonjourmauko]

Problem

There's no explicit contribution process, making it harder for contributors to propose fixes, and making the overall contributing experience rather confusing.

See /pull/68

Possible solution

It would be superb if could explain the expected contribution mechanism in a CONTRIBUTING.md file at the root of this repository.

See for example https://github.com/openfisca/openfisca-core/blob/master/CONTRIBUTING.md