README: Add sysext update service to Ignition setup

The systemd-sysupdate components need to be updated either manually or
automatically.
Add a timer unit to do automatic updates. The unit is based on the
upstream systemd-sysupdate.timer.

README.md

@@ -150,6 +150,31 @@ storage:
           Type=regular-file
           Path=/opt/extensions/docker
           CurrentSymlink=/etc/extensions/docker.raw
+systemd:
+  units:
+    - name: update-sysext.service
+      enabled: false
+      contents: |
+        [Unit]
+        Wants=network-online.target
+        After=network-online.target
+        [Service]
+        Type=oneshot
+        ExecStart=/usr/lib/systemd/systemd-sysupdate -C docker update
+        ExecStart=/usr/lib/systemd/systemd-sysupdate -C kubernetes update
+        [Install]
+        Also=update-sysext.timer
+    - name: update-sysext.timer
+      enabled: true
+      contents: |
+        [Timer]
+        OnBootSec=15min
+        OnUnitActiveSec=2h
+        OnCalendar=Sat
+        RandomizedDelaySec=4h
+        Persistent=yes
+        [Install]
+        WantedBy=timers.target
 ```
 
 ### Creating a custom Docker sysext image