docs: add badges

Author: robertsLando

README.md

@@ -2,6 +2,11 @@
 
 ![Github Test Status](https://github.com/mqttjs/MQTT.js/workflows/MQTT.js%20CI/badge.svg) [![codecov](https://codecov.io/gh/mqttjs/MQTT.js/branch/master/graph/badge.svg)](https://codecov.io/gh/mqttjs/MQTT.js)
 
+[![js-standard-style](https://img.shields.io/badge/code%20style-standard-brightgreen.svg?style=flat)](http://standardjs.com/) [![Maintenance](https://img.shields.io/badge/Maintained%3F-yes-green.svg)](https://github.com/mqttjs/MQTT.js/graphs/commit-activity)
+[![PRs Welcome](https://img.shields.io/badge/PRs-welcome-brightgreen.svg)](https://github.com/mqttjs/MQTT.js/pulls)\
+
+![NPM Version](https://img.shields.io/npm/v/mqtt?logo=npm) ![NPM Downloads](https://img.shields.io/npm/dm/mqtt.svg)
+
 MQTT.js is a client library for the [MQTT](http://mqtt.org/) protocol, written
 in JavaScript for node.js and the browser.