Contribution docs

This change will add the formal Contributor License Agreement (CLA) as well as documenting the Pull Request process for other contributions later.
Do we need the first sentence here? It's unclear to me what that means in relation to signing a CLA. Can' we just leave that out and say every dev has to sign it?

Also, `Every developer (even our own) to get started is required, sign the...` could be `Every developer (even our own) is required to sign the...`.