docs: add release notes

Signed-off-by: Nishant Bansal <nishant.bansal.282003@gmail.com>

Author: NishantBansal2003

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

@@ -75,6 +75,12 @@
   transitions during startup, avoiding lost unlocks during slow database
   initialization.
 
+* [Fixed the gossiper accepting malformed gossip
+messages](https://github.com/lightningnetwork/lnd/pull/10581) by enforcing chain
+hash validation on `ReplyChannelRange`, `ReplyShortChanIDsEnd` and
+`GossipTimestampRange`, and ensuring that if timestamps are provided in
+`ReplyChannelRange`, they are provided for each ShortChanID.
+
 # New Features
 
 - Basic Support for [onion messaging forwarding](https://github.com/lightningnetwork/lnd/pull/9868)