Merge pull request #3309 from rabbitmq/mk-3.9.4-release-notes

Initial 3.9.4 release notes

Author: michaelklishin

release-notes/3.9.4.md

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+RabbitMQ `3.9.4` 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 more
+consistent release schedule.
+
+### Core Server
+
+#### Enhancements
+
+ * Nodes will now use four more environment variables, if set: `RABBITMQ_DEFAULT_USER` (overrides `default_user` in `rabbitmq.conf`), `RABBITMQ_DEFAULT_PASS` (overrides `default_pass`), `RABBITMQ_DEFAULT_VHOST` (overrides `default_vhost`) and `RABBITMQ_ERLANG_COOKIE` (sets [shared authentication secret value](https://www.rabbitmq.com/clustering.html#erlang-cookie)).
+   These variables **are not recommended to be used in production** but can be the only realistic option in some environment, such as service containers, ECS, and so on.
+   Most users should continue using `rabbitmq.conf` and a securely generated local cookie file.
+
+   GitHub issue: [#3299](https://github.com/rabbitmq/rabbitmq-server/pull/3299)
+
+
+
+## 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-3.9.4.tar.xz` instead of the source tarball produced by GitHub.