Let release-managers create new branches

This will let them make major releases.

Change-Id: Ide788e80b07f129b2d9bc60293c821ede83671ab
Reviewed-on: https://go-review.googlesource.com/129215
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

project.config

diff --git a/project.config b/project.config
index c0e2b09..8843131 100644
--- a/project.config
+++ b/project.config
@@ -37,6 +37,7 @@
 [access "refs/heads/*"]
 	create = group admins
 	create = group gerritbot
+	create = group release-managers
 	forgeAuthor = group admins
 	forgeAuthor = group gerritbot
 	forgeAuthor = group may-forge-author-identity