Add codeclimate badge (#1572)

Co-authored-by: Leibale Eidelman <leibale1998@gmail.com>

README.md

@@ -15,6 +15,7 @@
     <a href="https://github.com/NodeRedis/node-redis/actions?query=workflow%3A%22Tests+Windows%22"><img src="https://github.com/NodeRedis/node-redis/workflows/Tests%20Windows/badge.svg" alt="Windows Build Status" /></a>
     <a href="https://coveralls.io/r/NodeRedis/node-redis?branch="><img src="https://coveralls.io/repos/NodeRedis/node-redis/badge.svg?branch=master" alt="Coverage Status" /></a>
     <a href="https://twitter.com/NodeRedis"><img src="https://img.shields.io/twitter/follow/NodeRedis.svg?style=flat-square&colorA=1da1f2&colorB=&label=Follow%20on%20Twitter" alt="Follow on Twitter"></a>
+    <a href="https://codeclimate.com/github/NodeRedis/node-redis/maintainability"><img src="https://api.codeclimate.com/v1/badges/f6d7063243c234237e73/maintainability" /></a>
     <a href="https://lgtm.com/projects/g/NodeRedis/node-redis/context:javascript"><img src="https://img.shields.io/lgtm/grade/javascript/g/NodeRedis/node-redis.svg?logo=lgtm&logoWidth=18" alt="Coverage Status" /></a>
 </p>