docs(readme): update requirements section to include webpack@4+

Author: shellscape

README.md

@@ -24,6 +24,11 @@ Some of the benefits of using this middleware include:
 has completed.
 - Supports hot module reload (HMR).
 
+## Requirements
+
+This module requires a minimum of Node v6.9.0 and Webpack v4.0.0, and must be used with a
+server that accepts express-style middleware.
+
 ## Getting Started
 
 First thing's first, install the module:
@@ -34,11 +39,6 @@ npm install webpack-dev-middleware --save-dev
 
 _Note: We do not recommend installing this module globally._
 
-## Requirements
-
-`webpack-dev-middleware` requires Node v6 or higher, and must be used with a
-server that accepts express-style middleware.
-
 ## Usage
 
 ```js
@@ -427,4 +427,4 @@ We welcome your contributions! Please have a read of [CONTRIBUTING.md](CONTRIBUT
 [middleware-url]: https://github.com/webpack/webpack-dev-middleware
 [stack-url]: https://stackoverflow.com/questions/tagged/webpack-dev-middleware
 [uglify-url]: https://github.com/webpack-contrib/uglifyjs-webpack-plugin
-[wjo-url]: https://github.com/webpack/webpack.js.org
+[wjo-url]: https://github.com/webpack/webpack.js.org