Display CII best practice badge in README

Fixes antrea-io#989

README.md

@@ -4,6 +4,7 @@
 
 ![Build Status](https://github.com/vmware-tanzu/antrea/workflows/Go/badge.svg?branch=master)
 [![Go Report Card](https://goreportcard.com/badge/github.com/vmware-tanzu/antrea)](https://goreportcard.com/report/github.com/vmware-tanzu/antrea)
+[![CII Best Practices](https://bestpractices.coreinfrastructure.org/projects/4173/badge)](https://bestpractices.coreinfrastructure.org/projects/4173)
 [![License](https://img.shields.io/badge/License-Apache%202.0-blue.svg)](https://opensource.org/licenses/Apache-2.0)
 ![GitHub release](https://img.shields.io/github/release/vmware-tanzu/antrea.svg?include_prereleases)