add CHANGELOG.md seeded from existing tags

[sujithps]

Summary

The repo has git tags (v0.1.1, v0.1.0, v0.1.0-alpha.4, v0.1.0-alpha.3) but no CHANGELOG.md and no published GitHub Releases, so followers get no human-readable history of what changed between versions.

Scope

• Add a top-level CHANGELOG.md in Keep a Changelog format.
• Seed one section per existing tag, summarizing changes from git log between tags (Added / Changed / Fixed).
• Add an ## [Unreleased] section at the top for ongoing work.
• Optional: a line in CONTRIBUTING.md asking PRs to add a changelog entry.

Acceptance criteria

• CHANGELOG.md covers every existing tag, newest first.
• Format validates against Keep a Changelog structure.

Note

Pairs with publishing GitHub Releases for the existing tags (a maintainer action, since it needs write access).

Proposed as a good first issue; a maintainer will apply the label.

[suchintan]

Landed in ef7984f (#145): CHANGELOG.md seeded from every published tag in Keep a Changelog format, with a guard that keeps the structure consistent going forward. Thanks!