Add release cycle information

Add release cycle information as discussed > https://github.com/mailcow/mailcow-dockerized/issues/1290

docs/install-update.md

@@ -83,3 +83,8 @@ Running `docker-compose down` - for example - will delete your containers. Your
 docker rmi -f $(docker images -f "dangling=true" -q)
 docker volume rm $(docker volume ls -qf dangling=true)
 ```
+
+
+Footnote
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+There is no release cycle regarding updates.