add badge for npm downloads (#818)

Author: spnraju

README.md

@@ -3,5 +3,7 @@
 [![npm version](https://badge.fury.io/js/nightwatch-api.svg)](https://badge.fury.io/js/nightwatch-api)
 [![Build Status](https://travis-ci.org/mucsi96/nightwatch-api.svg?branch=master)](https://travis-ci.org/mucsi96/nightwatch-api)
 [![Coverage Status](https://coveralls.io/repos/github/mucsi96/nightwatch-api/badge.svg?branch=master)](https://coveralls.io/github/mucsi96/nightwatch-api?branch=master)
+[![npm](https://img.shields.io/npm/dw/nightwatch-api?style=flat)](https://www.npmjs.com/package/nightwatch-api)
+[![github](https://img.shields.io/badge/PRs-welcome-blue.svg)](https://github.com/mucsi96/nightwatch-api)
 
 ## [Documentation](https://nightwatch-api.netlify.com)