Add release notes

Author: saikishor

doc/release_notes.rst

@@ -9,6 +9,7 @@ controller_interface
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 * The new ``MagneticFieldSensor`` semantic component provides an interface for reading data from magnetometers. (`#2627 <https://github.com/ros-controls/ros2_control/pull/2627>`__)
 * The controller_manager will now deactivate the entire controller chain if a controller in the chain fails during the update cycle. `(#2681 <https://github.com/ros-controls/ros2_control/pull/2681>`__)
+* The update rate of the controller will now be approximated to a closer achievable frequency, when its frequency is not achievable with the current controller manager update rate. (`#2828 <https://github.com/ros-controls/ros2_control/pull/2828>`__)
 
 controller_manager
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