Add CHANGELOG.md for release tracking

[CaptainDriftwood]

Description

Add a CHANGELOG.md file to track changes between versions following the Keep a Changelog format.

Recommended Format

# 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/1.1.0/),
and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).

## [Unreleased]

## [0.1.0] - YYYY-MM-DD

### Added
- Initial release
- Sync and async ICAP clients
- pytest plugin with fixtures
- Docker testing infrastructure

Automation Options

Consider using Release Drafter to auto-generate changelog entries from PR titles.

Priority

Low - Recommended before PyPI release.