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
diff --git a/doc/source/index.rst b/doc/source/index.rst
index 93fcc3a..b590380 100644
--- a/doc/source/index.rst
+++ b/doc/source/index.rst
@@ -98,6 +98,14 @@
 
    tempest_and_plugins_compatible_version_policy
 
+Stable Branch Testing Policy
+----------------------------
+
+.. toctree::
+   :maxdepth: 2
+
+   stable_branch_testing_policy
+
 Library
 -------