Add npm dependency checker (#253)

* add dependency badge

Author: mertkahyaoglu

README.md

@@ -1,6 +1,7 @@
 [![Gem Version](https://img.shields.io/gem/v/jekyll-admin.svg)](https://rubygems.org/gems/jekyll-admin)
 [![Build Status](https://travis-ci.org/jekyll/jekyll-admin.svg?branch=master)](https://travis-ci.org/jekyll/jekyll-admin)
 [![Build status](https://ci.appveyor.com/api/projects/status/biop1r6ae524xlm2/branch/master?svg=true)](https://ci.appveyor.com/project/benbalter/jekyll-admin/branch/master)
+[![NPM Dependencies](https://david-dm.org/jekyll/jekyll-admin.svg)](https://david-dm.org/jekyll/jekyll-admin)
 
 A Jekyll plugin that provides users with a traditional CMS-style graphical interface to author content and administer Jekyll sites. The project is divided into two parts. A Ruby-based HTTP API that handles Jekyll and filesystem operations, and a Javascript-based front end, built on that API.