README: Add sysext update service to Ignition setup

The systemd-sysupdate components need to be updated either manually or
automatically.
Extend the OS-level sysupdate timer to also cover additional components
and reload the sysext images after an update.

README.md

@@ -150,6 +150,24 @@ storage:
           Type=regular-file
           Path=/opt/extensions/docker
           CurrentSymlink=/etc/extensions/docker.raw
+systemd:
+  units:
+    - name: systemd-sysupdate.timer
+      enabled: true
+    - name: systemd-sysupdate.service
+      dropins:
+        - name: docker.conf
+          contents: |
+            [Service]
+            ExecStartPre=/usr/lib/systemd/systemd-sysupdate -C docker update
+        - name: kubernetes.conf
+          contents: |
+            [Service]
+            ExecStartPre=/usr/lib/systemd/systemd-sysupdate -C kubernetes update
+        - name: sysext.conf
+          contents: |
+            [Service]
+            ExecStartPost=systemctl restart systemd-sysext
 ```
 
 ### Creating a custom Docker sysext image