docs: add release note for premature msg fix

docs/release-notes/release-notes-0.15.0.md

@@ -18,6 +18,11 @@
   due to the cancel signal is processed before the creation. It is now properly
   handled by moving creation before deletion.   
 
+* When the block height+delta specified by a network message is greater than
+  the gossiper's best height, it will be considered as premature and ignored.
+  [These premature messages are now saved into a cache and processed once the
+  height has reached.](https://github.com/lightningnetwork/lnd/pull/6054)
+
 ## Misc
 
 * [An example systemd service file](https://github.com/lightningnetwork/lnd/pull/6033)