doc: Add release-notes for Kimai service

nixos/doc/manual/release-notes/rl-2405.section.md

@@ -61,6 +61,8 @@ In addition to numerous new and upgraded packages, this release has the followin
 
 <!-- Please keep entries alphabetically sorted. -->
 
+- [Kimai](https://www.kimai.cloud/), Kimai is a web-based multi-user time-tracking application. Works great for everyone: freelancers, companies, organizations - everyone can track their times, generate reports, create invoices and do so much more.
+
 - [Anki Sync Server](https://docs.ankiweb.net/sync-server.html), the official sync server built into recent versions of Anki. Available as [services.anki-sync-server](#opt-services.anki-sync-server.enable).
 The pre-existing [services.ankisyncd](#opt-services.ankisyncd.enable) has been marked deprecated and will be dropped after 24.05 due to lack of maintenance of the ankisyncd software.