add dependencies badge on readme (brianc#1024)

Author: a0viedo

README.md

@@ -2,6 +2,7 @@
 
 [![Build Status](https://secure.travis-ci.org/brianc/node-postgres.svg?branch=master)](http://travis-ci.org/brianc/node-postgres)
 ![io.js supported](https://img.shields.io/badge/io.js-supported-green.svg)
+[![Dependency Status](https://david-dm.org/brianc/node-postgres.svg)](https://david-dm.org/brianc/node-postgres)
 
 PostgreSQL client for node.js.  Pure JavaScript and optional native libpq bindings.