release_notes: update with Python ALPN fix

Author: guggero

release_notes.md

@@ -19,3 +19,7 @@ This file tracks release notes for the loop client.
 #### Breaking Changes
 
 #### Bug Fixes
+- Certain versions of the Python gRPC library
+  [weren't able to connect](https://github.com/grpc/grpc/issues/23172) to
+  `loopd`'s gRPC interface, getting the `missing selected ALPN property` error.
+  A server side fix was introduced to get rid of that error message.