systemd: Document how to add custom shutdown actions

As discussed in flatcar/Flatcar#65 the single
path that systemd supports for shutdown hooks does not work for read-
only images but we can use regular units to run custom shutdown
actions. Document one way to do it (there are many other ways how to
express this with systemd units).

content/docs/latest/setup/systemd/getting-started.md

@@ -157,6 +157,24 @@ ExecStartPost=/usr/bin/etcdctl set /domains/example.com/%H:%i running
 
 This gives us the flexibility to use a single unit file to announce multiple copies of the same container on a single machine (no port overlap) and on multiple machines (no hostname overlap).
 
+## Shutdown hooks
+
+While systemd allows to add custom hooks in `/usr/lib/systemd/system-shutdown/` that get run for `poweroff`/`halt`/`reboot`/`kexec` events, this path is not writable on Flatcar Container Linux. Therefore, regular units need to be used for running, e.g., a special cleanup action on shutdown:
+
+```ini
+[Unit]
+Description=Custom cleanup on shutdown
+DefaultDependencies=no
+After=final.target
+
+[Service]
+Type=oneshot
+ExecStart=bash -c 'echo bye; touch /bye'
+
+[Install]
+WantedBy=final.target
+```
+
 ## More information
 
 - [`systemd.service` Docs](http://www.freedesktop.org/software/systemd/man/systemd.service.html)