Move away from using staging and core-updates, and make the strategy
independant of branch names.
Keep the 300 dependent threshold for changes to master, as I don't have any
specific reason to change this.
Most importantly, require using guix-patches issues to coordinate merging of
the branches, as I think that'll address the key issues that have shown up
recently where it's been unclear which branch should be merged next.
* doc/contributing.texi (Submitting Patches): Move the branching strategy to a
new Managing Patches and Branches section.
(Managing Patches and Branches): New section.
(Commit Policy): Simplify through referencing the new Managing Patches and
Branches section.
Signed-off-by: Christopher Baines <mail@cbaines.net>