docs(readme): add npmjs link

Author: curbengh

README.md

@@ -1,6 +1,8 @@
 # hexo-generator-feed
 
-[![Build Status](https://travis-ci.org/hexojs/hexo-generator-feed.svg?branch=master)](https://travis-ci.org/hexojs/hexo-generator-feed)  [![NPM version](https://badge.fury.io/js/hexo-generator-feed.svg)](http://badge.fury.io/js/hexo-generator-feed) [![Coverage Status](https://img.shields.io/coveralls/hexojs/hexo-generator-feed.svg)](https://coveralls.io/r/hexojs/hexo-generator-feed?branch=master)
+[![Build Status](https://travis-ci.org/hexojs/hexo-generator-feed.svg?branch=master)](https://travis-ci.org/hexojs/hexo-generator-feed)
+[![NPM version](https://badge.fury.io/js/hexo-generator-feed.svg)](https://www.npmjs.com/package/hexo-generator-feed)
+[![Coverage Status](https://img.shields.io/coveralls/hexojs/hexo-generator-feed.svg)](https://coveralls.io/r/hexojs/hexo-generator-feed?branch=master)
 
 Generate Atom 1.0 or RSS 2.0 feed.