Adds the --save-dev flag to the install command (#49)

Author: siakaramalegos

README.md

@@ -22,7 +22,9 @@ The extract-loader works similar to the [extract-text-webpack-plugin](https://gi
 Installation
 ------------------------------------------------------------------------
 
-`npm install extract-loader`
+```bash
+$ npm install extract-loader --save-dev
+```
 
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