Changelog support should be documented

Apparently hackage has special support for Changelog files (in multiple formats?), which can be included directly in uploaded package tarballs. I can't find any documentation for this feature. https://hackage.haskell.org/upload doesn't even mention changelogs.

At minimum, some user visible page should say
- that changelogs are A Thing,
- what filenames hackage looks for, and
- what the supported formats are.

(See https://github.com/haskell/cabal/issues/10474 for the cabal side of things.)