Give example of testsuite driver file

[RyanGlScott]

This repo leaves out a description of (arguably) the most important part: how to actually use the generated build module in a testsuite! I don't think this needs to be much, just a simple description in a README that shows this code:

module Main where

import Build_doctests (flags, pkgs, module_sources)
import Data.Foldable (traverse_)
import Test.DocTest

main :: IO ()
main = do
    traverse_ putStrLn args
    doctest args
  where
    args = flags ++ pkgs ++ module_sources

[RyanGlScott]

Oops, I just noticed that you put in this in the code's comments itself.

But I think it'd be nice to have a README anyway - I can tackle this one.