Add release notes about speed scaling

Author: fmauch

doc/release_notes.rst

@@ -11,6 +11,10 @@ force_torque_sensor_broadcaster
 joint_trajectory_controller
 *******************************
 * The controller now supports the new anti-windup strategy of the PID class, which allows for more flexible control of the anti-windup behavior. (`#1759 <https://github.com/ros-controls/ros2_controllers/pull/1759>`__).
+* Scaling support was added in `#1191
+  <https://github.com/ros-controls/ros2_controllers/pull/1191>`__. With this the controller
+  "stretches the time" with which it progresses in the trajectory. Scaling can either be set
+  manually or it can be synchronized with the hardware. See :ref:`jtc_speed_scaling` for details.
 
 pid_controller
 *******************************

joint_trajectory_controller/doc/speed_scaling.rst

@@ -1,3 +1,5 @@
+.. _jtc_speed_scaling:
+
 Speed scaling
 =============