Suggest running `fedpkg retire` only in Rawhide
    
    Apparently, at some point the retirement process has changed slightly,
    and `fedpkg retire` is only needed for Rawhide. Simplify the
    instructions to suggest doing that, and relying on Git operations to
    duplicate the retirement commit for Branched, if needed. This approach
    is consitent with the approach that is also used in other places in
    Package Maintainer Docs, where basic Git operations are used to
    manipulate dist-git where possible, and `fedpkg` commands are only used
    when they perform some additional tasks.
    
    Fixes #148