Add breaking changes to the release notes

Author: ansd

release-notes/4.0.0.md

@@ -81,6 +81,14 @@ TBD
 
 TBD
 
+## Breaking Changes
+
+* RabbitMQ 3.13 [rabbitmq.conf](https://www.rabbitmq.com/docs/configure#config-file) settings `mqtt.default_user`, `mqtt.default_password`, and `amqp1_0.default_user` are unsupported in RabbitMQ 4.0.
+Instead, set the new RabbitMQ 4.0 settings `anonymous_login_user` and `anonymous_login_pass` (both values default to `guest`).
+For production scenarios, [disallow anonymous logins](https://www.rabbitmq.com/docs/next/production-checklist#anonymous-login).
+* RabbitMQ 3.13 `rabbitmq.conf` setting `rabbitmq_amqp1_0.default_vhost` is unsupported in RabbitMQ 4.0.
+Instead `default_vhost` will be used to determine the default vhost an AMQP 1.0 client connects to (i.e. when the AMQP 1.0 client does not define the vhost in the `hostname` field of the `open` frame).
+* RabbitMQ shovels can connect to a RabbitMQ 4.0 node via AMQP 1.0 only when the shovel runs on a RabbitMQ node >= 3.13.7
 
 ### Dependency Changes