release_notes: mention initiator field

Author: guggero

release_notes.md

@@ -16,6 +16,13 @@ This file tracks release notes for the loop client.
 
 #### NewFeatures
 
+* When requesting a swap, a new `initiator` field can be set on the gRPC/REST
+  interface that is appended to the user agent string that is sent to the server
+  to give information about Loop usage. The initiator field is meant for user
+  interfaces to add their name to give the full picture of the binary used
+  (`loopd`, LiT) and the method/interface used for triggering the swap (`loop`
+  CLI, autolooper, LiT UI, other 3rd party UI).
+
 #### Breaking Changes
 
 #### Bug Fixes