Update release notes

Signed-off-by: Mathias L. Baumann <[email protected]>

Author: Marenz

RELEASE_NOTES.md

@@ -11,6 +11,7 @@
 ## New Features
 
 - A new module `frequenz.client.dispatch.test` has been added, providing a fake Service and Client as well as a `DispatchGenerator` to generate `Dispatch` instances filled with random data.
+- The `DispatchGenerator.generate_dispatch` method now accepts a `microgrid_id` parameter to generate `Dispatch` instances with a specific microgrid ID.
 
 ## Bug Fixes