Add document for stable branch testing for tempest plugins

As Tempest and plugins master version is used to test the supported
stable branches, we need to test them on master gate.

Adding document for that so that we can test the stable branches
more consistently on plugins side too.

Change-Id: Ied3c6bb4032fd3740ae0d6d185e4a138fd335b02

doc/source/index.rst

@@ -98,6 +98,14 @@ Tempest & Plugins Compatible Version Policy
 
    tempest_and_plugins_compatible_version_policy
 
+Stable Branch Testing Policy
+----------------------------
+
+.. toctree::
+   :maxdepth: 2
+
+   stable_branch_testing_policy
+
 Library
 -------
 

doc/source/stable_branch_testing_policy.rst

@@ -0,0 +1,33 @@
+Stable Branch Testing Policy
+============================
+
+Tempest and its plugins need to support the stable branches
+as per :doc:`Stable Branch Support Policy </stable_branch_support_policy>`.
+
+Because of branchless model of Tempest and plugins, all the supported
+stable branches use the Tempest and plugins master version for their
+testing. That is done in devstack by using the `master branch
+<https://opendev.org/openstack/devstack/src/commit/c104afec7dd72edfd909847bee9c14eaf077a28b/stackrc#L314>`_
+for the Tempest installation. To make sure the master version of Tempest or
+plugins (for any changes or adding new tests) is compatible for all
+the supported stable branches testing, Tempest and its plugins need to
+add the stable branches job on the master gate. That way can test the stable
+branches against master code and can avoid breaking supported branches
+accidentally.
+
+Example:
+
+* `Stable jobs on Tempest master
+  <https://opendev.org/openstack/tempest/src/commit/e8f1876aa6772077f85f380677b30251c2454505/.zuul.yaml#L646-L651>`_.
+
+* `Stable job on neutron tempest plugins
+  <https://opendev.org/openstack/neutron-tempest-plugin/src/commit/4bc1b00213cf660648cad1916fe6497ac29b2e78/.zuul.yaml#L1427-L1428>`_
+
+Once any stable branch is moved to the `Extended Maintenance Phases`_
+and devstack start using the Tempest older version for that stable
+branch testing then we can remove that stable branch job from master
+gate.
+
+Example: https://review.opendev.org/#/c/722183/
+
+.. _Extended Maintenance Phases: https://docs.openstack.org/project-team-guide/stable-branches.html#extended-maintenance