chore: add a CHANGELOG.md

[RichardBJamison]

Now that we've tagged v1.0.0, a CHANGELOG.md following Keep a Changelog would make releases easy to follow.

Task

• Create CHANGELOG.md at the repo root
• Add an Unreleased section and a 1.0.0 entry summarizing the launch
• Link it from the README

Great first contribution.

[nightcityblade]

Hi, I'd like to work on this. I'll submit a PR shortly.