docs(readme): add Greenkeeper badge

https://greenkeeper.io/

Author: greenkeeper[bot]

README.md

@@ -1,5 +1,7 @@
 # fastify-redis
 
+[![Greenkeeper badge](https://badges.greenkeeper.io/fastify/fastify-redis.svg)](https://greenkeeper.io/)
+
 [![js-standard-style](https://img.shields.io/badge/code%20style-standard-brightgreen.svg?style=flat)](http://standardjs.com/)  [![Build Status](https://travis-ci.org/fastify/fastify-redis.svg?branch=master)](https://travis-ci.org/fastify/fastify-redis)
 
 Fastify Redis connection plugin, with this you can share the same Redis connection in every part of your server.