Add RawGit tip to docs

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/