Update release notes

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

Author: Marenz

RELEASE_NOTES.md

@@ -6,7 +6,7 @@
 
 ## Upgrading
 
-<!-- Here goes notes on how to upgrade from previous versions, including deprecations and what they should be replaced with -->
+* An API key for authorization must now be passed to the client.
 
 ## New Features