Add README information about virtualenv across clusters.
Add README information about virtualenv across clusters.

chambm/docker-galaxy-stable,afgane/docker-galaxy-stable,afgane/docker-galaxy-stable,bgruening/docker-galaxy-stable,chambm/docker-galaxy-stable,bgruening/docker-galaxy-stable,bgruening/docker-galaxy-stable,afgane/docker-galaxy-stable,chambm/docker-galaxy-stable