docs: add initial CHANGELOG.md using Keep a Changelog standard

[AaineeSinha]

Closes #10. Added a baseline CHANGELOG.md file to the repository root adhering to the Keep a Changelog standard.

[rosspeili]

Thanks for adding this @AaineeSinha, the setup looks good and is exactly what we needed to get started.

Before merge, could you tweak a couple things in the same PR?

• CHANGELOG.md, keep the structure, but add a short note that we’re still pre-release and versioned entries will come later (you can backfill old PRs in a short list if you want but not mandatory for this PR).
• README.md, link to CHANGELOG.md in the docs section.
• CONTRIBUTING.md, one line that the changelog exists; we’ll ask for [Unreleased] updates once we start doing releases.

That’s it, thanks again, and welcome to Rooms <3