Clarify the meaning of "triple licensed"

Also see: https://groups.google.com/g/rabbitmq-users/c/u5vxN9vWXTA/m/q6NevlMKEgAJ

@michaelklishin

Author: lukebakken

LICENSE

@@ -4,6 +4,9 @@ version 2 ("GPL") and the Apache License version 2 ("ASL"). For the MPL,
 please see LICENSE-MPL-RabbitMQ. For the GPL, please see LICENSE-GPL2.
 For the ASL, please see LICENSE-APACHE2.
 
+This means that you may choose one of these licenses when including or
+using this software in your own.
+
 The RabbitMQ Java client library includes third-party software under the
 ASL. For this license, please see LICENSE-APACHE2. For attribution of
 copyright and other details of provenance, please refer to the source code.