added changelog

Author: keith-decker

util/opentelemetry-util-genai/CHANGELOG.md

@@ -5,4 +5,13 @@ 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
+## [Unreleased]
+
+### Added
+
+- Generate Spans for LLM invocations
+- Generate Metrics for LLM invocations
+- Generate Logs for LLM invocations
+- Helper functions for starting and finishing LLM invocations
+
+