Mention AMQP durable field in 4.2 release notes

Author: ansd

release-notes/4.2.0.md

@@ -3,6 +3,21 @@
 RabbitMQ 4.2.0 is a new feature release.
 
 
+## Breaking Changes and Compatibility Notes
+
+### Default value for AMQP 1.0 `durable` field.
+
+Starting with RabbitMQ 4.2, if a sending client omits the header section, RabbitMQ [assumes](https://github.com/rabbitmq/rabbitmq-server/pull/13918) the `durable` field to be `false` complying with the AMQP 1.0 spec:
+```
+<field name="durable" type="boolean" default="false"/>
+```
+
+AMQP 1.0 apps or client libraries must set the `durable` field of the header section to `true` to mark the message as durable.
+
+Team RabbitMQ recommends client libraries to send messages as durable by default.
+All AMQP 1.0 client libraries [maintained by Team RabbitMQ](https://www.rabbitmq.com/client-libraries/amqp-client-libraries) send messages as durable by default.
+
+
 ## Features
 
 ### Incoming and Outgoing Message Interceptors for native protocols