Add a CHANGELOG.md

[collinsadi]

There is no CHANGELOG.md, and although the README references published hashes per release, the release notes only live in git history and the GitHub Releases tab. We should adopt Keep a Changelog and SemVer, seed it from the existing tags, and add a CONTRIBUTING note to update it per change.

Acceptance criteria

• CHANGELOG.md exists with an Unreleased section and the past releases.
• CONTRIBUTING references the update expectation.