Create `Contributing` documentation

[trym-b]

Motivation

If others want to contribute code to our repositories, we need to have to make sure we have the necessary legal rights to the code, otherwise changing licenses, etc could be legally dubious.

Feature

Add a CONTRIBUTING.md file that contains how to contribute, with a Contributor Licence Agreement that must be signed before contributing. An option is to base it on grpc-go contributing guidelines: https://github.com/grpc/grpc-go/blob/master/CONTRIBUTING.md