How to implement screenfull.js?

I want to use screenfull.js which will display my entire stage in fullscreen mode like videos do.

But I cannot make it work.

Here is the source:

https://github.com/sindresorhus/screenfull.js/

And here is the demo:

http://sindresorhus.com/screenfull.js/

I want to get the same result as when clicked on the 'request document' button in the demo.

Can someone help me out?

Thanks in advance!