Add RawGit tip to docs

[graham-sportsmgmt]

I needed to add a library from git to my twiddle and received the following error in the browser console:

Refused to execute script from 'https://raw.githubusercontent.com/willowsystems/jSignature/master/src/jSignature.js' because its MIME type ('text/plain') is not executable, and strict MIME type checking is enabled.

A good solution to this problem is to use rawgit as a proxy for the file, perhaps a note should be added to the docs?

http://rawgit.com/

[Gaurav0]

Yes. We need docs. :)