Add 4.2.0 release notes

[skip ci]

Author: ansd

release-notes/4.2.0.md

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+## RabbitMQ 4.2.0
+
+RabbitMQ 4.2.0 is a new feature release.
+
+
+## Features
+
+### Incoming and Outgoing Message Interceptors for native protocols
+
+Incoming and outgoing messages can now be intercepted on the broker.
+This works for AMQP 1.0, AMQP 0.9.1, and MQTT.
+
+What the interceptor does is entirely up to its implementation - it can validate message metadata, add annotations, or perform arbitrary side effects.
+Custom interceptors can be developed and integrated via [plugins](./plugins).
+
+Two new optional built-in interceptors were added to RabbitMQ:
+1. Timestamps for outgoing messages
+2. Setting client ID of publishing MQTT client
+
+Detailed information can be found in the [Message Interceptor](https://www.rabbitmq.com/docs/next/message-inteceptor) documentation.