Add CHANGELOG

Author: ocelotl

CHANGELOG.md

@@ -7,6 +7,8 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## Unreleased
 
+- Fix explicit bucket histogram aggregation
+  ([#3429](https://github.com/open-telemetry/opentelemetry-python/pull/3429))
 - Remove Jaeger exporters
   ([#3554](https://github.com/open-telemetry/opentelemetry-python/pull/3554))
 - Log stacktrace on `UNKNOWN` status OTLP export error