Add contribution guidelines

[liamsi]

Besides the usual foo, clarify which parts of the API are supposed to be non-breaking. Additionally, clarify how contributors can still propose breaking changes (and what increases their chances to land such changes in this library).
If there are any relevant specs contributors should be aware of, these should be mentioned in the contribution guidelines, too.

[musalbas]

Any changes that break the tests would probably be breaking the API.

[universe2439]

Agree