Update `CHANGELOG` for `0.2.0` release

[voxpelli]

Its a bit empty in regards to the 0.2.0 release in the extension page in VS Code:

No biggie, but thought I could just as well post a nudge about it as its neat to get to know that a lovely footnotes extension is now included 🎉

Also, OT-question: Any reason why this extension is at a 0.x release line rather than a stable >=1.x line? See https://semver.org/#spec-item-4