Add CHANGELOG

Author: xrmx

CHANGELOG.md

@@ -17,6 +17,8 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
   ([#4371](https://github.com/open-telemetry/opentelemetry-python/pull/4371))
 - Fix span context manager typing by using ParamSpec from typing_extensions
   ([#4389](https://github.com/open-telemetry/opentelemetry-python/pull/4389))
+- Fix serialization of None values in logs body to match 1.31.0+ data model
+  ([#4400](https://github.com/open-telemetry/opentelemetry-python/pull/4400))
 - [BREAKING] semantic-conventions: Remove `opentelemetry.semconv.attributes.network_attributes.NETWORK_INTERFACE_NAME`
   introduced by mistake in the wrong module.
   ([#4391](https://github.com/open-telemetry/opentelemetry-python/pull/4391))