Add browserify usage instructions to docs

highlight.js works like a charm with Browserify, but it took me a while to figure out how. Eventually I found https://github.com/isagalaev/highlight.js/pull/195 which got me on the right track.

I've added a bit of documentation so others don't have so much trouble as I did. I'm not sure this file is the right place for it. let me know if I should move it, or if it would make more sense to add it to the highlightjs.org website source somewhere.