Update Webpack instructions to accommodate new rules

[MainShayne233]

First off, thank you so much for this guide! I found it incredibly helpful with setting up an Electron + Elm app.

Faced a few issues with some of the webpack steps, and wanted to help anyone else from tripping over them.

Also removed sudo from the npm install calls since it is ill- advised:

If you get an EACCES error, you should fix your permissions. You could also try using sudo, but this should be avoided