docs(readme) WP4 deprecation for css. Link to MiniCSSExtractPlugin (#777)

Author: EugeneHlushko

README.md

@@ -29,6 +29,8 @@ npm install --save-dev [email protected]
 
 <h2 align="center">Usage</h2>
 
+> :warning: Since webpack v4 the `extract-text-webpack-plugin` should not be used for css. Use [mini-css-extract-plugin](https://github.com/webpack-contrib/mini-css-extract-plugin) instead.
+
 > :warning: For webpack v1, see [the README in the webpack-1 branch](https://github.com/webpack/extract-text-webpack-plugin/blob/webpack-1/README.md).
 
 ```js