Add dependency status badge (#33)

Author: thiamsantos

README.md

@@ -3,7 +3,7 @@
   <br>
   <b>Mitt</b>: tiny 200b functional event emitter / pubsub.
   <br>
-  <a href="https://www.npmjs.org/package/mitt"><img src="https://img.shields.io/npm/v/mitt.svg?style=flat" alt="npm"></a> <a href="https://travis-ci.org/developit/mitt"><img src="https://travis-ci.org/developit/mitt.svg?branch=master" alt="travis"></a>
+  <a href="https://www.npmjs.org/package/mitt"><img src="https://img.shields.io/npm/v/mitt.svg?style=flat" alt="npm"></a> <a href="https://travis-ci.org/developit/mitt"><img src="https://travis-ci.org/developit/mitt.svg?branch=master" alt="travis"></a> <a href="https://david-dm.org/developit/mitt"><img src="https://david-dm.org/developit/mitt/status.svg" alt="dependencies Status"></a>
 </p>
 
 ## Why Mitt?