Add breaking change to release notes

(cherry picked from commit 7ada1b8)

Author: ansd

release-notes/4.0.0.md

@@ -117,6 +117,13 @@ RabbitMQ 3.13 `rabbitmq.conf` setting `rabbitmq_amqp1_0.default_vhost` is unsupp
 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).
 
+Starting with RabbitMQ 4.0, RabbitMQ strictly validates that
+[delivery annotations](https://docs.oasis-open.org/amqp/core/v1.0/os/amqp-core-messaging-v1.0-os.html#type-delivery-annotations),
+[message annotations](https://docs.oasis-open.org/amqp/core/v1.0/os/amqp-core-messaging-v1.0-os.html#type-message-annotations), and
+[footer](https://docs.oasis-open.org/amqp/core/v1.0/os/amqp-core-messaging-v1.0-os.html#type-footer) contain only
+[non-reserved annotation keys](https://docs.oasis-open.org/amqp/core/v1.0/os/amqp-core-messaging-v1.0-os.html#type-annotations).
+As a result, clients can only send symbolic keys that begin with `x-`.
+
 ### MQTT
 
 RabbitMQ 3.13 [rabbitmq.conf](https://www.rabbitmq.com/docs/configure#config-file) settings `mqtt.default_user`, `mqtt.default_password`,