Add doc for supported OpenStack release & py version

Tempest is branchless and support many OpenStack stable branches
and python versions. Users of Tempest should have a clear doc about
what all openstack releases and python versions are supported.

Change-Id: Idc76e07efdd3710a59c773c564ca532419989ed5
diff --git a/doc/source/index.rst b/doc/source/index.rst
index 7acfd62..ab994d1 100644
--- a/doc/source/index.rst
+++ b/doc/source/index.rst
@@ -48,6 +48,14 @@
    workspace
    run
 
+Supported OpenStack Releases and Python Versions
+------------------------------------------------
+
+.. toctree::
+   :maxdepth: 1
+
+   supported_version
+
 Developers Guide
 ================