Add design document for using AD users/groups in SUDO rules
    
    Fixes: https://pagure.io/freeipa/issue/3226
    Signed-off-by: Alexander Bokovoy <abokovoy@redhat.com>
    Reviewed-By: Christian Heimes <cheimes@redhat.com>
    Reviewed-By: Rob Crittenden <rcritten@redhat.com>
    Reviewed-By: Florence Blanc-Renaud <frenaud@redhat.com>