write content for webpack tutorial

I don't know much about Webpack, but I would love to have a section on the new Istanbul website that helps walk beginners through the process of setting up their project for coverage:

there's a section just waiting for content:

http://istanbul.js.org/2016/08/02/webpack.html

@iam4x, @JaKXz do you think you could help pull something together for beginners?