Add design draft
    
    The design draft explains implementation details, limitations, and API
    changes for the new feature.
    
    Fixes: https://pagure.io/freeipa/issue/7892
    Signed-off-by: Christian Heimes <cheimes@redhat.com>
    Reviewed-By: Thomas Woerner <twoerner@redhat.com>
    Reviewed-By: Francois Cami <fcami@redhat.com>