doc: Design document for LDAP graceperiod
    
    Implement part of RFC https://tools.ietf.org/id/draft-behera-ldap-password-policy-10.html
    
    Related: https://pagure.io/freeipa/issue/1539
    
    Signed-off-by: Rob Crittenden <rcritten@redhat.com>
    Reviewed-By: Alexander Bokovoy <abokovoy@redhat.com>
    Reviewed-By: Florence Blanc-Renaud <frenaud@redhat.com>