Create u_e-checkmk.en.md

docs/manual-guides/checkmk/u_e-checkmk.en.md

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+Mailcow provides the ability to check for updates using its own update script.
+
+If you want to check for mailcow updates using checkmk, you can create an executable file in the `local` directory of the checkmk agent (typically `/usr/lib/check_mk_agent/local/`) with the name `mailcow_update` and the following content:
+
+````
+#!/bin/bash
+cd /opt/mailcow-dockerized/ && ./update.sh -c >/dev/null
+status=$?
+if [ $status -eq 3 ]; then
+  echo "0 \"mailcow_update\" mailcow_update=0;1;;0;1 No updates available."
+elif [ $status -eq 0 ]; then
+  echo "1 \"mailcow_update\" mailcow_update=1;1;;0;1 Updated code is available.\nThe changes can be found here: https://github.com/mailcow/mailcow-dockerized/commits/master"
+else
+  echo "3 \"mailcow_update\" - Unknown output from update script ..."
+fi
+exit
+````
+
+If the mailcow installation directory is not `/opt/`, adjust this in the 2nd line.
+
+After that re-inventory the services for your mailcow host in checmk and a new check named `mailcow_update` should be selectable.
+
+## Screenshots
+
+### No updates available
+
+If there are no updates available, `OK` is displayed.
+
+![No update available](../../assets/images/checkmk/no_updates_available.png)
+
+### New updates available
+
+If updates are available, `WARN` is displayed.
+
+![Updates available](../../assets/images/checkmk/updates_available.png)
+
+If `CRIT` is desired instead, replace the 7th line with the following:
+
+````
+  echo "1 \"mailcow_update\" mailcow_update=1;1;;0;1 Updated code is available.\nThe changes can be found here: https://github.com/mailcow/mailcow-dockerized/commits/master"
+````
+
+### Detailed check output
+
+![Long check output](../../assets/images/checkmk/long_check_output.png)
+
+- This provides a link to mailcow's GitHub commits, if updates are available.
+- Metrics are also displayed ( not only when updates are available):
+  - 0 = No updates available
+  - 1 = New updates available