[Dependency: Guides] - Add v4 Plugin and Loader Migration example - Place in Migrating/4 - Post MVP

[TheLarkInn]

Dependency for #1859 (POST MVP)

As a plugin author, I would like to know how to upgrade my webpack loader or plugin to support version 4.

• Inside of the webpack.js.org/migrate/4 guide, we should include (at the bottom of the content), how to upgrade a webpack loader and plugin.
• Instructions should resemble the same diff style as rest of document
• This part of the guide should link back to our Loader and Plugin Reference Implementation