Tag deliverables as retired

From the release team point of view we need to ensure that deliverables are
well tagged as retired to ensure that our machinery will work as
expected all along the series life cycle.

Here is an example of experienced problem with your validation:

https://review.opendev.org/c/openstack/releases/+/792820/5..6

Change-Id: I6cb59e7cc09bd6a8cda0ffe06ffcd896696aea86

doc/source/repository.rst

@@ -72,8 +72,31 @@ list in the file ``tools/www-generator.py`` in the
 the URL ``https://docs.openstack.org/openstack/<projectname>/latest``
 will redirect to the repositories' ``README.rst`` file.
 
+Step 6: Mark the deliverables as retired
+----------------------------------------
 
-Step 6: Remove Repository from the Governance Repository
+For maintained openstack series, on ``openstack/releases``, amend the related
+deliverable files to tag the project with the ``retired`` flag.
+
+Example with ``deliverables/train/puppet-panko.yaml``::
+
+    ---
+    launchpad: puppet-panko
+    release-model: cycle-trailing
+    team: Puppet OpenStack
+    type: other
+    repository-settings:
+      openstack/puppet-panko:
+        flags:
+          - retired
+    ...
+
+Even if a project is retired, stable branches will continue to follow the
+existing series life cycle and this flag will allow us to ignore this
+deliverable in some specific cases.
+
+
+Step 7: Remove Repository from the Governance Repository
 --------------------------------------------------------
 
 Remove the repository from the ``reference/projects.yaml`` file and