Add a changelog file.

Author: michaelsafyan

instrumentation-genai/opentelemetry-instrumentation-google-genai/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
+
+Create an initial version of Open Telemetry instrumentation for github.com/googleapis/python-genai.
+
+This version adds support for instrumenting `Client.models.generate_content` but does not yet support
+instrumentation for streaming and async use cases.
+
+This version supports some of the `gen_ai.request.*` span attributes but does not cover all of the
+GenAI semantic conventions for spans that are applicable just yet.