readme: dev setup needs to have fedmsg-hub in venv
    
    It's complicated, but unfortunately there seems to be no easier way
    (if you don't want to overwrite your system files). Also, fedmsg
    must be installed in the system because of
    https://github.com/fedora-infra/fedmsg/issues/399 .
    
    Differential Revision: https://phab.qa.fedoraproject.org/D1154