Prepare release notes

Signed-off-by: cwasicki <[email protected]>

Author: cwasicki

RELEASE_NOTES.md

@@ -2,6 +2,11 @@
 
 ## Summary
 
+This is the initial release of the Frequenz Client Common Library.
+It provides wrappers around the types from Frequenz's common API protobuf definitions.
+This version contains types to support pagination in gRPC requests
+and enum types for components and metrics to be used in a microgrid.
+
 <!-- Here goes a general summary of what this release is about -->
 
 ## Upgrading