Note to add 'jinja2' and 'markupsafe' to requirements

Change-Id: I14d80d353084d54d8e114b10524e5b46660cf34b
Closes-Bug: #1552704

CONTRIBUTING.rst

@@ -24,3 +24,9 @@ Pull requests submitted through GitHub will be ignored.
 Bugs should be filed on Launchpad, not GitHub:
 
    https://bugs.launchpad.net/openstack-manuals
+
+.. note::
+
+   To be able to run ``"tox -e py27"`` successfully locally, add
+   ``jinja2`` and ``markupsafe`` to your local ``test-requirements.txt``
+   file so the two get installed in your local virtual environment.