add cii badge

README.md

@@ -4,6 +4,7 @@
 [![Docker Repository on Quay](https://quay.io/repository/prometheus/prometheus/status)][quay]
 [![Docker Pulls](https://img.shields.io/docker/pulls/prom/prometheus.svg?maxAge=604800)][hub]
 [![Go Report Card](https://goreportcard.com/badge/github.com/prometheus/prometheus)](https://goreportcard.com/report/github.com/prometheus/prometheus)
+[![CII Best Practices](https://bestpractices.coreinfrastructure.org/projects/486/badge)](https://bestpractices.coreinfrastructure.org/projects/486)
 
 Visit [prometheus.io](https://prometheus.io) for the full documentation,
 examples and guides.