add docs

docs/40_Usage/40_Cronjob.md

@@ -44,3 +44,18 @@ $ cat /etc/cron.d/client-backup
 7 18 * * 6 root /usr/local/bin/cronwrapper.sh 1440 /opt/imlbackup/client/prune.sh 'iml-backup-prune'
 7 18 * * 0 root /usr/local/bin/cronwrapper.sh 1440 /opt/imlbackup/client/verify.sh 'iml-backup-verify'
 ```
+
+## Desktop notification
+
+If you don't run the backup on a server but on a client you can activate the desktop notification for a user that is logged in.
+Set a variable `SUPER_USER=<USERNAME>` in your cron file.
+
+Example:
+
+```shell
+$ cat /etc/cron.d/client-backup 
+
+SUPER_USER=axel
+
+*/17 * * * root /usr/local/bin/cronwrapper.sh 1440 /opt/imlbackup/client/backup.sh 'iml-backup'
+```