Document our approach on going to Standard style

We are currently converting the code base into Standard style by converting each line that we touch when making changes. This approach is very nice since it doesn't introduce any additional conflicts and makes backporting work well.

However, I've never seen this actually documented somewhere. It would be nice if we could write down what we are doing, and why we are doing it. Maybe in the contributor guide.

We're following the same approach in [node-canvas](https://github.com/Automattic/node-canvas) and I would love to be able to link to the description from there 😄