Add documentation for modular composes
    
    List all available configuration options and go into some detail on how
    modules should be specified. This would probably deserver a more
    thorough explanation, but it's still bit in a flux.
    
    Fixes: https://pagure.io/pungi/issue/767
    Signed-off-by: Lubomír Sedlář <lsedlar@redhat.com>