Update motd.sls

This patch unifies /etc/motd managing approach for both RedHat and
Ubuntu systems. Providing a string value via linux:system:motd
pillar will configure static /etc/motd and remove dynamic scripts
from /etc/update-motd.d (if present).

update-motd can safely be removed because Ubuntu supports dynamic
motd by pam_motd means since 2009.

Related-Prod: PROD-17287

Change-Id: Ic9b7e18abb12cfe8704717b14dc1237e40715319
diff --git a/README.rst b/README.rst
index fea2e1a..2492aaf 100644
--- a/README.rst
+++ b/README.rst
@@ -825,9 +825,28 @@
 Message of the day
 ~~~~~~~~~~~~~~~~~~
 
-``pam_motd`` from package ``update-motd`` is used for dynamic messages of the
+``pam_motd`` from package ``libpam-modules`` is used for dynamic messages of the
 day. Setting custom motd will cleanup existing ones.
 
+Setting static motd will replace existing ``/etc/motd`` and remove scripts from
+``/etc/update-motd.d``.
+
+Setting static motd:
+
+.. code-block:: yaml
+
+    linux:
+      system:
+        motd: |
+          UNAUTHORIZED ACCESS TO THIS SYSTEM IS PROHIBITED
+
+          You must have explicit, authorized permission to access or configure this
+          device. Unauthorized attempts and actions to access or use this system may
+          result in civil and/or criminal penalties.
+          All activities performed on this system are logged and monitored.
+
+Setting dynamic motd:
+
 .. code-block:: yaml
 
     linux: