Add EventsExecutor release notes (#3662)

* Add EventsExecutor release notes

Signed-off-by: Michael Carroll <[email protected]>
Signed-off-by: Chris Lalancette <[email protected]>
Co-authored-by: Christophe Bedard <[email protected]>
(cherry picked from commit 07cc792)

source/Releases/Release-Iron-Irwini.rst

@@ -232,6 +232,32 @@ To make this a bit better, there is now a new ``NodeInterfaces`` class that can
 
 There are examples on how to use this in https://github.com/ros2/rclcpp/pull/2041.
 
+Introduction of a new executor type: the Events Executor
+""""""""""""""""""""""""""""""""""""""""""""""""""""""""
+
+The ``EventsExecutor`` from iRobot has been merged into the main ``rclcpp`` codebase.
+This alternative executor implementation uses event-driven callbacks from the middleware implementations to fire callbacks at the ``rclcpp`` layer.
+In addition to the push-based model, the ``EventsExecutor`` also moves timer management into a separate thread, which can allow for more accurate results and lower overhead, especially with many timers.
+
+The ``EventsExecutor`` has a substantial set of documentation and use-in-practice that make it a strong candidate for inclusion in the ``rclcpp`` codebase.
+For information about the initial implementation proposal as well as performance benchmarks, see https://discourse.ros.org/t/ros2-middleware-change-proposal/15863.
+For more information about the design, see the design PR: https://github.com/ros2/design/pull/305.
+
+Since the API is the same, trying the ``EventsExecutor`` is as straightforward as replacing your current Executor implementation (eg. ``SingleThreadedExecutor``):
+
+.. code-block:: C++
+
+    #include <rclcpp/experimental/executors/events_executor/events_executor.hpp>
+    using rclcpp::experimental::executors::EventsExecutor;
+
+    EventsExecutor executor;
+    executor.add_node(node);
+    executor.spin();
+
+**Note** The ``EventsExecutor`` and ``TimersManager`` are currently in the ``experimental`` namespace.
+While it has been used as a standalone implementation for some time https://github.com/irobot-ros/events-executor, it was decided to use the ``experimental`` namespace for at least one release to give latitude in changing the API within the release.
+Use caution as it will not be subject to the same API/ABI guarantees that the non-experimental code has.
+
 ``rclpy``
 ^^^^^^^^^