release-process: document schedule (including freezes) publicly

Change-Id: Ic037b54630017740d7859292b32b8c57f4af7854
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/274772


Tested-by: Mike Frysinger <vapier@google.com>
Reviewed-by: Jonathan Nieder <jrn@google.com>