add a badge to indicate docker image pull number.

[TECO-7]

README.md

@@ -2,6 +2,7 @@
 [![Build status](https://ci.appveyor.com/api/projects/status/kf3pwh2or5afsgl9/branch/master?svg=true)](https://ci.appveyor.com/project/sangshuduo/tdengine-2n8ge/branch/master)
 [![Coverage Status](https://coveralls.io/repos/github/taosdata/TDengine/badge.svg?branch=develop)](https://coveralls.io/github/taosdata/TDengine?branch=develop)
 [![CII Best Practices](https://bestpractices.coreinfrastructure.org/projects/4201/badge)](https://bestpractices.coreinfrastructure.org/projects/4201)
+![Docker Pulls](https://img.shields.io/docker/pulls/tdengine/tdengine)
 
 [![TDengine](TDenginelogo.png)](https://www.taosdata.com)