Document __webpack_nonce__

[quantizor]

There's a clever bit of functionality built into webpack for Content Security Policy purposes, enabled via __webpack_nonce__. It'd be great to formally document it and give examples for why it's useful.

https://github.com/webpack/webpack/blob/ab2270263e9af5b0dd83e454c20ab3aa1797424a/lib/APIPlugin.js#L26

https://stackoverflow.com/questions/42922784/what-s-the-purpose-of-the-html-nonce-attribute-for-script-and-style-elements