Add changelog

Author: pyohannes

CHANGELOG.md

@@ -20,6 +20,8 @@ release.
   ([#615](https://github.com/open-telemetry/semantic-conventions/pull/615))
 - Add an example for gcp_pubsub asynchronous batch publish
   ([#545](https://github.com/open-telemetry/semantic-conventions/pull/545)).
+- Add messaging semantic conventions for settlement spans
+  ([#661](https://github.com/open-telemetry/semantic-conventions/pull/661)).
 
 ### Fixes