Add note about ros2topic auto and now keywords (ros2#2998)

* Add note about ros2topic auto and now keywords

Signed-off-by: Esteve Fernandez <[email protected]>
Co-authored-by: Chris Lalancette <[email protected]>

Author: esteve

source/Releases/Release-Iron-Irwini.rst

@@ -41,7 +41,16 @@ To come.
 New features in this ROS 2 release
 ----------------------------------
 
-To come.
+ros2topic
+^^^^^^^^^
+
+`now` as keyword for `builtin_interfaces.msg.Time` and `auto` for `std_msgs.msg.Header`
+"""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""
+`ros2 topic pub` now allows to set a `builtin_interfaces.msg.Time` message to the current time via the `now` keyword.
+Similarly, a `std_msg.msg.Header` message will be automatically generated when passed the keyword `auto`.
+This behavior matches that of ROS 1's `rostopic` (http://wiki.ros.org/ROS/YAMLCommandLine#Headers.2Ftimestamps)
+
+Related PR: `ros2/ros2cli#749 <https://github.com/ros2/ros2cli/pull/749>`_
 
 Changes since the Humble release
 ----------------------------------