add instructions for goodparts badge to readme

[Nelson]

add instructions for how to include the goodparts badge

[![JavaScript Style Guide: Good Parts](https://img.shields.io/badge/code%20style-goodparts-brightgreen.svg?style=flat)](https://github.com/dwyl/goodparts "JavaScript The Good Parts")

and a link to https://github.com/dwyl/goodparts so people know why we wrote our "own" style...!

relates to: dwyl/goodparts#253