Mark creating new branch under user and projects needs to inform git-admin first

Similarly, it will make the maintenance svn2git convertor easier when it's
getting close to the flag day of migrating src repository to git.

This is only for creating new branches under user and projects, the existing
branches are still open for development needs before repository being migrated
to git. Please commit with "Approved by: git-admin (implicitly)"

More details:
https://github.com/bsdimp/freebsd-git-docs/blob/main/projects-user.md

Approved by: core
Sponsored by: The FreeBSD Foundation