Add CII Best Practices badge

Signed-off-by: Stefan Prodan <stefan.prodan@gmail.com>

README.md

@@ -1,5 +1,6 @@
 # Flux version 2
 
+[![CII Best Practices](https://bestpractices.coreinfrastructure.org/projects/4782/badge)](https://bestpractices.coreinfrastructure.org/projects/4782)
 [![e2e](https://github.com/fluxcd/flux2/workflows/e2e/badge.svg)](https://github.com/fluxcd/flux2/actions)
 [![report](https://goreportcard.com/badge/github.com/fluxcd/flux2)](https://goreportcard.com/report/github.com/fluxcd/flux2)
 [![license](https://img.shields.io/github/license/fluxcd/flux2.svg)](https://github.com/fluxcd/flux2/blob/main/LICENSE)