Add note for Content-Length header in the documentation

[xavierfoucrier]

When dealing with videos, the preloader need to get the size of the video from the server through the Content-Length header, otherwise the onprogress event will not be fired properly due to this instruction:

https://github.com/andreupifarre/preload-it/blob/608d7c6c6706f933925a824ef2859b414c63dfbf/src/preloadOne.js#L6

This is important and should be specified in the documentation.