docs: update release notes for 0.21.0

Author: gijswijs

docs/release-notes/release-notes-0.21.0.md

@@ -22,6 +22,12 @@
 # Bug Fixes
 
 # New Features
+
+- Basic Support for [onion messaging forwarding](https://github.com/lightningnetwork/lnd/pull/9868) 
+  consisting of a new message type, `OnionMessage`. This includes the message's
+  definition, comprising a path key and an onion blob, along with the necessary
+  serialization and deserialization logic for peer-to-peer communication.
+
 ## Functional Enhancements
 
 ## RPC Additions