Fix fot tf forwarding.

communication.md

@@ -66,7 +66,7 @@ ROS2 agent:
   to are prepended with the prefix "/ext". That means that all
   programs generating messages that is also intended for external
   agents need to prepend "/ext" to the topic. The **mqtt_to_ros**
-  program will push out the message on the topic without a prefix.
+  program will push out the message on the topic without a prefix (except for /ext/tf).
   This program can also be configured to reduce the number of messages
   it sends by having a stride for each topic. Also for each topic we
   can choose to send it to ALL agents or to a specific agent and we