docs: update CONTRIBUTING.md with some PR setup info (#2619)
docs: update CONTRIBUTING.md with some PR setup info (#2619)

Adds an explicit mention to enable "Allow edits by maintainers" when
setting up the pull request.  In PR #2574 (external contribution) it was
discovered this cannot be added after the fact for org-based forks, only
user-based.

As there's not a great reference link for this in the github docs that
doesn't descend into a bunch of organization permissions management, I'm
not calling this specific circumstance explicitly.
googleapis/google-cloud-go,googleapis/google-cloud-go,googleapis/google-cloud-go,googleapis/google-cloud-go