updating CSS docs to reference main.css (#2079)
updating CSS docs to reference main.css (#2079)

Also, o need to dupe this documentation over two repos.
JohnnyWalkerDesign/html5-boilerplate,JohnnyWalkerDesign/html5-boilerplate,h5bp/html5-boilerplate,rprouse/html5-boilerplate,h5bp/html5-boilerplate,roblarsen/html5-boilerplate,roblarsen/html5-boilerplate,rprouse/html5-boilerplate