Update release notes

Signed-off-by: Leandro Lucarella <[email protected]>

RELEASE_NOTES.md

@@ -6,7 +6,9 @@
 
 ## Upgrading
 
-<!-- Here goes notes on how to upgrade from previous versions, including deprecations and what they should be replaced with -->
+- This release stops using `betterproto` and `grpclib` as backend for gRPC and goes back to using Google's `grpcio` and `protobuf`.
+
+    If your code was using `betterproto` and `grpclib`, it now needs to be ported to use `grpcio` and `protobuf` too. Remember to also remove any `betterproto` and `grpclib` dependencies from your project.
 
 ## New Features