Add a note about python2.6 support to the README

This commit adds a new section to the tempest README regarding python2.6
with tempest. It also outlines the steps required to enable testr with
python2.6.

Change-Id: I1603b24a9654ac5e71a54f63754d874cd39ce4da
Merge branch 'master' of https://github.com/openstack/tempest