Add CHANGELOG

Author: xrmx

CHANGELOG.md

@@ -15,6 +15,8 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 - `opentelemetry-instrumentation-botocore` Add support for GenAI user events and lazy initialize tracer
   ([#3258](https://github.com/open-telemetry/opentelemetry-python-contrib/pull/3258))
+- `opentelemetry-instrumentation-botocore` Add support for GenAI system events
+  ([#3266](https://github.com/open-telemetry/opentelemetry-python-contrib/pull/3266))
 
 ### Fixed