convert cabal-install/changelog to markdown

[juhp]

What is wrong with the docs?
The cabal-install changelog file is basically a list of links to the release files.
But it is a plain text file, so one has to copy urls and paste them in order to open them easily.

Converting to a simple markdown file changelog.md should be straightforward.
I could probably help with that: is the file just written by hand?

[ulysses4ever]

Dup of #8857? Contributions welcome!