CDN usage instructions

[tomByrer]

#569
I believe 99% of devs know how to use CDNs; I usually just submit a pull request with a direct link to the CDN archive. But I can see reveal.js attracting a wider audience, such as office workers & graphic designers who barely know how to hack HTML.

I believe CDNs can help speed up websites, & would be essential if a HTML file is sent via email (asking a non-technical executive to open a .zip file & expect him/her to click on the right file is asking a bit too much in my experience).

[hakimel]

See #569