Add npm badge

Author: crissdev

README.md

@@ -1,6 +1,8 @@
 # knockout.mapping
 
 [![Build Status](https://travis-ci.org/crissdev/knockout.mapping.svg?branch=master)](https://travis-ci.org/crissdev/knockout.mapping)
+[![npm version](https://badge.fury.io/js/knockout-mapping.svg)](http://badge.fury.io/js/knockout-mapping)
+
 
 > Object mapping plugin for [Knockout](http://knockoutjs.com/)