Update Release Notes

Signed-off-by: camille-bouvy-frequenz <[email protected]>

Author: camille-bouvy-frequenz

RELEASE_NOTES.md

@@ -16,6 +16,7 @@
 * Replace the local `PaginationParams` type with the `frequenz-client-common` one
 * Remove dependency to `googleapis-common-protos`
 * Replace `Energy` with `Power` for the `quantity` representation
+* Add str function for `DeliveryPeriod` object
 
 ## Bug Fixes