nixos/prometheus-mqtt-exporter: add release notes entry

Signed-off-by: Christoph Heiss <[email protected]>

nixos/doc/manual/release-notes/rl-2411.section.md

@@ -156,6 +156,8 @@
 
 - [MenhirLib](https://gitlab.inria.fr/fpottier/menhir/-/tree/master/coq-menhirlib) A support library for verified Coq parsers produced by Menhir.
 
+- [mqtt-exporter](https://github.com/kpetremann/mqtt-exporter/), a Prometheus exporter for exposing messages from MQTT. Available as [services.prometheus.exporters.mqtt](#opt-services.prometheus.exporters.mqtt.enable).
+
 - [Glance](https://github.com/glanceapp/glance), a self-hosted dashboard that puts all your feeds in one place. Available as [services.glance](option.html#opt-services.glance).
 
 - [Apache Tika](https://github.com/apache/tika), a toolkit that detects and extracts metadata and text from over a thousand different file types. Available as [services.tika](option.html#opt-services.tika).