CHANGELOG.md: Bring them back please

`CHANGELOG.md` was removed in #1911 because they fell out of date ([history](https://github.com/facebook/relay/commits/main/CHANGELOG.md)).

Proposal: Releases such as [v13.0.0](https://github.com/facebook/relay/releases/tag/v13.0.0) can be posted to `CHANGELOG.md` and the future git tags should be sync'd to them.

Changelog:
- Can be set to tags, e.g. [v0.10.0](https://github.com/facebook/relay/blob/v0.10.0/CHANGELOG.md), [v0.9.3](https://github.com/facebook/relay/blob/v0.9.3/CHANGELOG.md)
- Deep linking: [v0.10.0 ref links to v0.6.1](https://github.com/facebook/relay/blob/v0.10.0/CHANGELOG.md#061-january-8-2016)
- Can select lines of markdown to highlight changes, [highlighting 0.8.0 to 0.9.2](https://github.com/facebook/relay/blob/v0.10.0/CHANGELOG.md?plain=1#L28-L93)
- Command-F/Ctrl-F easily
- Scrolling

Releases:
- UX is like a blog system, not a changelog
- `CHANGES` files are one example where _we don't need a special UX revamp_, `CHANGES` work perfectly fine as they are
- Type setting and spacing, as opposed to be condensed and utilitarian, sacrifices utility for fitting the general gh aesthetic.
- If many releases behind, can easily end up paginating
- GitHub dependent

I don't think it's ever too late to start again CHANGELOG.md again.