add npm badge

Author: Alexander Ivantsov

README.md

@@ -2,6 +2,7 @@
 
 [![Build Status](https://travis-ci.org/ivantsov/redux-webext.svg?branch=master)](https://travis-ci.org/ivantsov/redux-webext)
 [![Coverage Status](https://coveralls.io/repos/github/ivantsov/redux-webext/badge.svg?branch=master)](https://coveralls.io/github/ivantsov/redux-webext?branch=master)
+[![npm version](https://badge.fury.io/js/redux-webext.svg)](https://badge.fury.io/js/redux-webext)
 
 This package allows you to use [Redux](https://github.com/reactjs/redux) for managing the state of your WebExtension.