Changelogs for packages aren't documented

**What is wrong with the docs?**
According to a user in `#hackage:matrix.org`, nowhere is it documented that Hackage requires packages to have a changelog file, nor what this involves.

**Additional context**
```
[24 09:40:50] the other day hackage complained that my package doesn't have a changelog
[24 09:41:03] what is a changelog, where does hackage look for it, and where is this documented?
[24 09:52:13] mauke: https://en.wikipedia.org/wiki/Changelog
[24 09:52:43] CHANGELOG.md is a good place to have your changelog
[24 09:53:36] and I believe it has to be put in https://cabal.readthedocs.io/en/stable/cabal-package-description-file.html#pkg-field-extra-doc-files
[24 09:53:47] mauke: run "cabal check" I think it'll tell you more
[24 09:54:17] cabal check just says "No errors or warnings could be found in the package."
[24 10:34:37] I can't find changelog anywhere in the cabal documentation. Or hackage.
[24 10:54:58] mauke: please tell us your "cabal --version"
[24 10:59:45] This is the pr #8497 (https://github.com/haskell/cabal/pull/8747) (alas, missing a changelog itself!)
```