Changelog support should be documented

[mauke]

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 haskell/cabal#10474 for the cabal side of things.)