Policy for repository use in the wrpc-sw, general-cores and wr-cores projects

1. All developers work against a branch called "proposed_master".
2. New features are developed either in a local proposed_master or in a branch and only pushed to ohwr/proposed_master when they are done+tested.
3. The repository maintainer moves proposed_master=>master when it seems in a good state.
4. When a release happens, a branch of master is created by the repository maintainer at that time.
5. The repository maintainer backports critical bug-fixes from proposed_master to the released branch.

Thanks to Wesley Terpstra for proposing the policy on which this one is largely based.