Add a changelog for OSV.dev

[jess-lowe]

Low priority but OSV.dev is missing a changelog

[github-actions[bot]]

This issue has not had any activity for 60 days and will be automatically closed in two weeks

See https://github.com/google/osv.dev/blob/master/CONTRIBUTING.md for how to contribute a PR if you're interested in helping out.

[ashmod]

should this be closed? (or does it require a changelog page/section on the website for example?)

[another-rex]

Yep, we have a CHANGELOG.md now