docs(openai-agents): add CHANGELOG.md to barebones package

Author: nagkumar91

instrumentation-genai/opentelemetry-instrumentation-openai-agents/CHANGELOG.md

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+# Changelog
+
+All notable changes to this project will be documented in this file.
+
+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
+
+- Initial barebones package skeleton: minimal instrumentor stub, version module,
+  and packaging metadata/entry point.