Added 'rel' attribute to stylesheet link in usage doc

This is vitally important - at least, it is in Chrome v29. Without it, the browser doesn't know to interpret that link as a stylesheet, and doesn't load the styles. And without those styles, Medium.js just doesn't work. Took me a lot of messing about to realise that; maybe I'm not the only one to copy the example from usage and assume that it works! ;-)