Adds Docker Pulls Badge to Readme

Author: erwanlr

README.md

@@ -15,6 +15,7 @@
 
 <p align="center">
   <a href="https://badge.fury.io/rb/wpscan" target="_blank"><img src="https://badge.fury.io/rb/wpscan.svg"></a>
+  <a href="https://hub.docker.com/r/wpscanteam/wpscan/" target="_blank"><img src="https://img.shields.io/docker/pulls/wpscanteam/wpscan.svg"></a>
   <a href="https://github.com/wpscanteam/wpscan/actions?query=workflow%3ABuild" target="_blank"><img src="https://github.com/wpscanteam/wpscan/workflows/Build/badge.svg"></a>
   <a href="https://codeclimate.com/github/wpscanteam/wpscan" target="_blank"><img src="https://codeclimate.com/github/wpscanteam/wpscan/badges/gpa.svg"></a>
 </p>