Add a doc about authenticating against v3
Add a doc about authenticating against v3

i've had to explain this too many times, and paste the env. vars
i'm using to folks on irc.

Change-Id: I87677c57b309a865c1bd8ea42dc44c00d3ec0489

redhat-openstack/python-openstackclient,dtroyer/python-openstackclient,BjoernT/python-openstackclient,openstack/python-openstackclient,dtroyer/python-openstackclient,BjoernT/python-openstackclient,openstack/python-openstackclient,redhat-openstack/python-openstackclient