Mention wekan in installation instructions

docs/installation_instructions.rst

@@ -374,6 +374,7 @@ to install:
 
 - Nextcloud and Onlyoffice with ``install-app.sh nextcloud``
 - Rocket.Chat with ``install-app.sh rocketchat``
+- Wekan with ``install-app.sh wekan``
 - WordPress with ``install-app.sh wordpress``
 - Velero with ``install-app.sh velero`` (only if you have configured it in
   :ref:`backups-with-velero`).