Revise and update README.md
    
    The information in this file was wildly outdated. This overhauls
    the whole thing to be accurate and updated (I hope). Note I've
    synced https://fedoraproject.org/wiki/How_to_use_and_edit_comps.xml_for_package_groups
    with the current state of this pull request.
    
    Signed-off-by: Adam Williamson <awilliam@redhat.com>