CSS Guiding Principles & Best Practices

@julienbenatar @mbildner @spucci @samirrayani @karlsluis @abigailricarte @adamhajari @dodeca Please review and comment.
Yes, it's part of a CSS naming system call BEM—short for Block, Element, Modifier. I'll cover the details in a separate document, but check out [this article](http://webdesign.tutsplus.com/articles/an-introduction-to-the-bem-methodology--cms-19403?utm_source=CSS-Weekly&utm_campaign=Issue-100&utm_medium=email) for a nice summary about it.