Add npm badge

Author: simplesmiler

README.md

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 > Reusable clickaway directive for reusable [Vue.js](https://github.com/vuejs/vue) components
 
+[![npm version](https://img.shields.io/npm/v/vue-clickaway.svg)](https://www.npmjs.com/package/vue-clickaway)
+
 ## Overview
 
 Sometimes you need to detect clicks **outside** of the element (to close a modal