Better explanation for style rules

[rstacruz]

"take a look at a sample file" is a pretty lazy way to explain the style rules. i've started to maintain a better documentation on the rules:

https://github.com/rstacruz/standard-doc

perhaps we should consider merging them into standard?

[feross]

This is excellent. I merged your document into RULES.md file and added you as a GitHub collaborator so you can modify the file as you see fit 👍

[feross]

Published to http://standardjs.com/rules.html