docs(guides): Added info for v4 and webpack-cli (#1888)

Author: kingdaro

src/content/guides/installation.md

@@ -28,6 +28,12 @@ npm install --save-dev webpack
 npm install --save-dev webpack@<version>
 ```
 
+If you're using webpack 4 or later, you'll also need to install the CLI.
+
+``` bash
+npm install --save-dev webpack-cli
+```
+
 Installing locally is what we recommend for most projects. This makes it easier to upgrade projects individually when breaking changes are introduced. Typically webpack is run via one or more [npm scripts](https://docs.npmjs.com/misc/scripts) which will look for a webpack installation in your local `node_modules` directory:
 
 ```json