node-sass dependency

node-sass is not beeing installed with sass-loader, so it has to be
added to the command

Author: Paweł Jasiołek

docs/configurations/pre-processors.md

@@ -7,7 +7,7 @@ In Webpack, all pre-processors need to be applied with a corresponding loader. `
 For example, let's compile our `<style>` tag with SASS:
 
 ``` bash
-npm install sass-loader --save-dev
+npm install sass-loader node-sass --save-dev
 ```
 
 ``` html