Add a changelog to `x.py`

[jyn514]

This would make it easier to find and learn more about breaking changes (or even just new features). It would also give us something to point to after a change, especially for changes not large enough to need their own blog post on inside-rust.

I'm imagining it would go in src/bootstrap/CHANGELOG.md, following https://keepachangelog.com/.

[jyn514]

As a related feature, x.py should say something when the version changes asking you to read the changelog. One possible idea is to have current-version = "x" at the top of config.toml and print a warning until you update it to the latest version.