Add release notes for scheduler bug fixes

Signed-off-by: Mathias L. Baumann <mathias.baumann@frequenz.com>

Author: Marenz

RELEASE_NOTES.md

@@ -20,3 +20,8 @@
 ## Bug Fixes
 
 <!-- Here goes notable bug fixes that are worth a special mention or explanation -->
+
+* Fixed bugs that could cause a dispatch actor not to stop when an update
+  arrives exactly at the moment a dispatch window closes. Also fixed a
+  related issue that could cause incorrect event ordering when multiple
+  dispatches are updated concurrently.