Add info about installation of dev dependencies to README

Adresses #219

Author: mischah

README.md

@@ -52,6 +52,12 @@ You can install jquery-placeholder by using [Bower](http://bower.io).
 bower install jquery-placeholder
 ```
 
+Contributors should install the »dev dependencies« after forking and cloning via [npm](https://www.npmjs.com/).
+
+```bash
+npm install
+```
+
 ## Notes
 
 * Requires jQuery 1.6+. For an older version of this plugin that works under jQuery 1.4.2+, see [v1.8.7](https://github.com/mathiasbynens/jquery-placeholder/tree/v1.8.7).