loopd: update release notes

Author: GeorgeTsagk

release_notes.md

@@ -16,6 +16,15 @@ This file tracks release notes for the loop client.
 
 #### New Features
 
+* Sweep Batcher: A new sub-system was added that handles all the loopout
+sweeps. Successful loopout HTLCs will no longer be swept back to the wallet via
+individual transactions but will instead form a single transaction that holds
+multiple inputs and pays to a single output. This will significantly reduce
+chain fee costs as it's using less block space by directly consolidating all the
+htlcs to a single address. Loopouts that pay to non-wallet addresses will still
+use individual transactions as their output cannot be mutated.
+
+
 #### Breaking Changes
 
 #### Bug Fixes