Add a changelog

We've been merging a lot of features lately and it would be nice to have something I could throw at https://this-week-in-rust.org.

We can start recording features from when the changelog is added to avoid too much pain. It would be a good first contribution to go back through the history and find when features were first added, but we shouldn't block on that.

Mentoring instructions: create a file called CHANGELOG.md following the format in https://keepachangelog.com/en/1.1.0/. Feel free to add as many or as few past features as you have time for. There is a list of merged PRs at https://github.com/rust-lang/docs.rs/pulls?q=is%3Apr+is%3Aclosed. Docs.rs doesn't have releases, so one possible way to organize this is by date, since deploys are not tracked in a central location.

Please comment here if you start working on this so we don't have multiple people working on it at the same time.

Read the Keep a Changelog 1.1.0 format linked in the issue, then review the closed pull request list for features worth recording. Create CHANGELOG.md with a date-based structure and any past features you choose to include; the work is done when the file follows the format and provides a usable record for future updates.