Include #14546 to the 4.2.0 release notes

Author: ansd

release-notes/4.2.0.md

@@ -213,6 +213,10 @@ This section is **incomplete** and will be expanded as 4.2 approaches its releas
    # (including the protocol-standard pre-declared ones) in the cluster
    cluster_exchange_limit = 200
    ```
+ * Routing via the [fanout exchange](https://www.rabbitmq.com/docs/exchanges#fanout) got optimised.
+   For messages published to the fanout exchange, end-to-end message throughput increases by up to 42%.
+
+   GitHub issue: [#14546](https://github.com/rabbitmq/rabbitmq-server/pull/14546)
 
 #### Bug Fixes