README update

groundskeeper doesn't play that nicely with minified code.
doing this makes sure that `groundskeeper-brunch` is run before in brunch's optimization step.

I got a problem where I had some code which, when obfuscated was `[..],console.log('something'),[..]` and groundskeeper removed the `console.log` but kept both the commas, resulting in a syntax error when run.