Tweaks to the pre-commit hook
Tweaks to the pre-commit hook

* Abort the commit if stylish-haskell returns non-zero.
* Improve documentation at the top of the file.

input-output-hk/cardano-sl,input-output-hk/pos-haskell-prototype,input-output-hk/cardano-sl,input-output-hk/cardano-sl,input-output-hk/pos-haskell-prototype,input-output-hk/cardano-sl,input-output-hk/pos-haskell-prototype