Fix a mistake in cbits and add section to README for hlint

[shayne-fletcher]

This is a good pre-push commit hook:

#!/bin/bash

for api in GHCLIB_API_808 GHCLIB_API_810 GHCLIB_API_811
do
  stack exec hlint -- --cpp-include cbits --cpp-define $api .
done

stack exec weeder -- . --build