Add a releasenote for additional_matcher

It really should have gone in with the review that implemented the
feature but better late than never.

Change-Id: Iafcfd17b9ddad8c09d66aadb6eda6f5603c23b8b

releasenotes/notes/additional-matcher-5c5cd466a6d70080.yaml

@@ -0,0 +1,4 @@
+---
+features:
+  - Allow specifying an `additional_matcher` to the mocker that will call a
+    function to allow a user to add their own custom request matching logic.