Add instructions for using moment-timezone with webpack

[gilmoreorless]

This is pretty much a copy of the webpack usage docs at https://momentjs.com/docs/#/use-it/webpack, but with a different note about how to reduce the size of the bundled data.

This partially resolves moment/moment-timezone#715, but doesn't mention the ES module syntax. I figured that adding ES usage should be a separate pull request, as it affects more than just webpack usage.

(Disclosure: I wrote the webpack plugin that’s mentioned in the page, so this is sort-of self promotion...)

[marwahaha]

Thanks!