CONTRIBUTING: Explain how to propose changes to the release branch

CONTRIBUTING.rst

@@ -27,9 +27,12 @@ To contribute patches:
 #. Run `Utilities/SetupForDevelopment.sh`_ for local configuration.
 #. See the `CMake Source Code Guide`_ for coding guidelines.
 #. Base all new work on the upstream ``master`` branch.
+   Base work on the upstream ``release`` branch only if it fixes a
+   regression or bug in a feature new to that release.
 #. Create commits making incremental, distinct, logically complete changes.
 #. Push a topic branch to a personal repository fork on GitLab.
-#. Create a GitLab Merge Request targeting the upstream ``master`` branch.
+#. Create a GitLab Merge Request targeting the upstream ``master`` branch
+   (even if the change is intended for merge to the ``release`` branch).
 
 The merge request will enter the `CMake Review Process`_ for consideration.