Update Foxy release notes. (ros2#704)

Signed-off-by: Michel Hidalgo <[email protected]>

Author: hidmic

source/Releases/Release-Foxy-Fitzroy.rst

@@ -227,6 +227,13 @@ Tools timestamp messages using ROS time
 
 Related pull request: https://github.com/ros2/rviz/pull/519
 
+Known Issues
+------------
+
+* `[ros2/ros2#922] <https://github.com/ros2/ros2/issues/922>`_ Services' performance is flaky for ``rclcpp`` nodes using eProsima Fast-RTPS or ADLINK CycloneDDS as RMW implementation.
+  Specifically, service clients sometimes do not receive the response from servers.
+
+
 Timeline before the release
 ---------------------------