Add an example on how to use webpack with webextensions

webpack-modules/README.md

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+# WebExtension Webpack Example
+A minimal example of how to use npm modules from within a WebExtension.
+The example package used by this extension is `left-pad`, an essential package
+in almost any situation.
+
+## What it does
+This setup takes a background and a content script entrypoint and outputs them
+to their respective target.
+
+## What it could do
+This could be infinitely extended - injecting global jQuery, adding babel,
+react/jsx, css modules, image processing and so on.
+
+## What it shows
+
+ - How to use npm or custom modules in a WebExtension.
+
+## How to build it
+
+ - `npm install`
+ - `npm run build`
+
+The WebExtension in the [dist](dist/) folder should now work.
+
+## What about Browserify?
+Browserify works just as well as webpack for extensions, in the end it's a
+personal choice about your prefered tool.
+
+## Live-development
+Additionally to watching the folder with your `manifest.json` in it, you will also
+have to run webpack in watch mode.