Add npm badge to readme file

Author: socsieng

README.md

@@ -1,6 +1,7 @@
 # typedoc-plugin-typescript-declaration
 
 [![Build Status](https://travis-ci.org/socsieng/typedoc-plugin-typescript-declaration.svg?branch=master)](https://travis-ci.org/socsieng/typedoc-plugin-typescript-declaration)
+[![NPM Version](https://badge.fury.io/js/typedoc-plugin-typescript-declaration.svg)](https://badge.fury.io/js/typedoc-plugin-typescript-declaration)
 
 This is a Typedoc plugin that renders a TypeScript declaration file.