This is so that our PR's don't suddenly start failing when the restyled.io service bumps is `fourmolu` version.
## TODO
I wasn't actually able to install version `0.10.1.0`; both of these attempts failed:
stack install fourmolu-0.10.1.0
and
cabal install fourmolu-0.10.1.0
Restyled.io uses fourmolu version `0.10.1.0`. It is not clear how it could be pegged to `0.4.0.0`. The latest I was able to install on my machine with `cabal install` was `0.9.0.0`.
fixes#888
- note how to install Fourmolu and run it from shell
- refactor the conventions to use subsections for easier readability
- simplify restyled arguments for Fourmolu 0.4