Add ChangeLog.md and NOTICE files to %doc
    
    The ChangeLog file was added in 1.6.0 and contains useful information
    for users.
    
    The AUTHORS file was removed from %doc in znc-1.7.0-1, because the file
    was no longer present in the upstream tarball.  Prior to that, the
    AUTHORS file was a symlink to the (unpackaged) NOTICES file from
    znc-1.2-0.1.alpha1 through znc-1.6.6.