Update README.md

Author: runjuu

README.md

@@ -3,7 +3,9 @@
 [![PRs Welcome](https://img.shields.io/badge/PRs-welcome-brightgreen.svg)](https://github.com/Runjuu/html-inline-css-webpack-plugin/pulls)
 [![npm version](https://badge.fury.io/js/html-inline-css-webpack-plugin.svg)](https://badge.fury.io/js/html-inline-css-webpack-plugin)
 
-Convert `<link rel="stylesheet"/>` into `<style>...<style/>`
+Convert external style sheet(`<link rel="stylesheet"/>`) to internal style sheet(`<style>...<style/>`).
+
+require [mini-css-extract-plugin](https://github.com/webpack-contrib/mini-css-extract-plugin) and [html-webpack-plugin](https://github.com/jantimon/html-webpack-plugin)
 
 ## Install
 #### NPM