docs(readme): add an example how to @import a node module (webpack-contrib#777)

* Add example of @import'ing a node module

The syntax for importing a node_module is not easy to find (see webpack/webpack#1789 (comment)). Maybe this will help a few people out.

* Incorporate PR Suggestions

Author: danny-andrews-snap

README.md

@@ -115,6 +115,12 @@ url(~module/image.png) => require('module/image.png')
 
 ### `import`
 
+To import styles from a node module path, prefix it with a `~`:
+
+```css
+@import '~module/styles.css';
+```
+
 To disable `@import` resolving by `css-loader` set the option to `false`
 
 ```css