mention CSS system design rationale in CONTRIBUTING.md

Created by: cvrebert

Specifically, I propose linking to http://markdotto.com/2012/02/16/scope-css-classes-with-prefixes/ and http://markdotto.com/2012/03/02/stop-the-cascade/

X-Ref: #11794 (closed) and some similar past issues/comments/questions.

/cc @mdo