Update readme

README.md

@@ -3,6 +3,10 @@ Kanboard
 
 Kanboard is project management software that focuses on the Kanban methodology.
 
+**This software is in maintenance mode**.
+It doesn't mean it's abandoned, but there is no significant feature development.
+Pull-requests are still accepted as long as the [guidelines](.github/pull_request_template.md) are followed.
+
 - Official website: <https://kanboard.org/>
 - [List of features](https://kanboard.org/#features)
 - [Change Log](https://github.com/kanboard/kanboard/blob/master/ChangeLog)