Add a Webpacker v4 Upgrade Guide to docs.

Credit to @justin808 for the original v4 upgrade guide this is based on.

docs/v4-upgrade.md

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+To update a Webpacker v3.5 app to v4, follow these steps:
+
+1. Update the `webpacker` gem and the `@rails/webpacker` package to v4. This will upgrade webpack itself from 3.x to 4.x, make sure you're aware of [any deprecations which might effect you](https://webpack.js.org/migrate/4/). Also make sure any other packages you depend on support webpack 4 and don't require any changes, e.g. if you explicitly include `webpack` you need to upgrade it to 4.x, and if you use `webpack-dev-server` you need to upgrade it to 3.x.
+1. Browser support definitions have been moved to [`.browserslistrc`](../lib/install/config/.browserslistrc) to `/`.
+1. Merge any differences between [`config/webpacker.yml`](../lib/install/config/webpacker.yml) and your `config/webpacker.yml`.
+1. Webpacker v4 upgrades Babel to [v7](https://babeljs.io/docs/en/v7-migration), see also [the release blog post](https://babeljs.io/blog/2018/08/27/7.0.0). Many packages were moved to the `@babel/` namespace, any babel plugins you have will need to be updated. It may be worth checking out [babel-upgrade](https://github.com/babel/babel-upgrade) if you have problems. ([#1564](https://github.com/rails/webpacker/pull/1564))
+1. `.babelrc` should be replaced with `babel.config.js` and `.postcssrc.yml` should be replaced with `postcss.config.js` ([#1822](https://github.com/rails/webpacker/pull/1822)). If you never changed these files from their defaults, the versions of [babel.config.js](../lib/install/config/babel.config.js) and [postcss.config.js](../lib/install/config/postcss.config.js) in the webpacker repository should be usable.
+1. Due to the change in [#1625](https://github.com/rails/webpacker/pull/1625), you'll want to make sure that `extract_css` is set to true for the `default` environment in `webpacker.yml` if you want to have Webpacker supply your CSS.
+
+Package-specific notes:
+
+- If you're using React, you need to add `"@babel/preset-react"`, to the list of `presets` in your babel config.
+- If you're using Vue Loader, you'll need to upgrade to [v15](https://vue-loader.vuejs.org/migrating.html) for webpack 4.
+- To see what webpacker generates for a given framework with v4, you may want to re-run `bundle exec rake webpacker:install:FRAMEWORK` and let it override the files for your given JavaScript framework, and then compare them to see what changes you'll need to make.
+
+This is what an upgrade to Webpacker 4 looked like for existing Rails apps (please contribute yours!):
+
+- https://github.com/connorshea/ContinueFromCheckpoint/pull/77