Add 3.9.6 release notes

I've missed adding notes to the release part of
#3340

I am going to add a new item to the PULL_REQUEST_TEMPLATE.md checklist
so that we get reminded about it next time.

Signed-off-by: Gerhard Lazu <gerhard@lazu.co.uk>

release-notes/3.9.6.md

@@ -0,0 +1,38 @@
+RabbitMQ `3.9.6` is a maintenance release in the `3.9.x` release series.
+
+Please refer to the **Upgrading to 3.9** section from [v3.9.0 release notes](https://github.com/rabbitmq/rabbitmq-server/releases/tag/v3.9.0) if upgrading from a version prior to 3.9.0.
+
+This release requires at least Erlang 23.2, and supports the latest Erlang 24 version, 24.0.5 at the time of release. [RabbitMQ and Erlang/OTP Compatibility Matrix](https://www.rabbitmq.com/which-erlang.html) has more details on Erlang version requirements for RabbitMQ.
+
+
+
+## Changes Worth Mentioning
+
+Release notes are kept under [rabbitmq-server/release-notes](https://github.com/rabbitmq/rabbitmq-server/tree/v3.9.x/release-notes).
+Contributors are encouraged to update them together with their changes. This helps with release automation and a more consistent release schedule.
+
+
+### Core Server
+
+#### Bug Fixes
+
+
+### Stream Plugin
+
+#### Bug Fixes
+
+ * When stream clients close connections abruptly, publisher and consumer metrics get cleaned up correctly.
+
+   GitHub issue: [#3340](https://github.com/rabbitmq/rabbitmq-server/pull/3340)
+
+
+
+## Dependency Upgrades
+
+No dependency changes in this release.
+
+
+
+## Source Code Archives
+
+To obtain source code of the entire distribution, please download the archive named `rabbitmq-server-3.9.6.tar.xz` instead of the source tarball produced by GitHub.