docs: add CHANGELOG.md

[usernane]

Problem

No changelog exists. Users have no way to know what changed between versions without reading commit history.

Suggestion

Add a CHANGELOG.md following the Keep a Changelog format.

Structure

# Changelog

All notable changes to this project will be documented in this file.

## [Unreleased]

## [2.1.9] - 2023-07-19
### Fixed
- ...

### Added
- ...

## [2.1.8] - ...

Expected Outcome

• Retroactively document known changes from git history
• All future releases update the changelog before tagging