rl-2505: mention openthread-border-router

nixos/doc/manual/release-notes/rl-2505.section.md

@@ -43,6 +43,8 @@
 
 - [MaryTTS](https://github.com/marytts/marytts), an open-source, multilingual text-to-speech synthesis system written in pure Java. Available as [services.marytts](options.html#opt-services.marytts).
 
+- [OpenThread Border Router](https://github.com/openthread/ot-br-posix), a border router bridging Thread mesh networks with IPv6 networks. Available as [services.openthread-border-router](#opt-services.openthread-border-router.enable).
+
 - [networking.modemmanager](options.html#opt-networking.modemmanager) has been split out of [networking.networkmanager](options.html#opt-networking.networkmanager). NetworkManager still enables ModemManager by default, but options exist now to run NetworkManager without ModemManager.
 
 - [Conduwuit](https://conduwuit.puppyirl.gay/), a federated chat server implementing the Matrix protocol, forked from Conduit. Available as [services.conduwuit](#opt-services.conduwuit.enable).