Add a CONTRIBUTING.md #69
Description
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