Improve text-only readme rendering

[gbaz]

As per: https://mail.haskell.org/pipermail/haskell-cafe/2018-April/128898.html the pre block is pretty ugly, especially with the new layout.

An easy fix would be to have pre-blocks (in general, or only when they are the whole readme?) use a whitespace style of "pre-wrap" to allow wrapping.

[phadej]

pre-wrap will make "formatted" (e.g. justified) text super ugly.

[NunoAlexandre]

The text size of the readme is huge, that should also be addressed.