add python-requests to README
    
    If we don't have python-requests then we can't import
    collect_atomic_vars from atomic_vars.py and it will silently
    not collect any atomic vars.
    
        /home/vagrant/fedora-websites/build.d/build.py(55)<module>()
        -> from atomic_vars import collect as collect_atomic_vars
        (Pdb)
        ImportError: 'No module named requests'