Update CHANGELOG.md

Author: jack-burridge-cfh

CHANGELOG.md

@@ -6,6 +6,8 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
 ## Unreleased
+- Move Protobuf encoding to its own package
+  ([#3169](https://github.com/open-telemetry/opentelemetry-python/pull/3169))
 - Change ``__all__`` to be statically defined.
   ([#3143](https://github.com/open-telemetry/opentelemetry-python/pull/3143))
 - Remove the ability to set a global metric prefix for Prometheus exporter