chore: add SLSA badge (#13282)

Signed-off-by: Justin Marquis <34fathombelow@protonmail.com>

README.md

@@ -1,6 +1,7 @@
 **Releases:**
 [![Release Version](https://img.shields.io/github/v/release/argoproj/argo-cd?label=argo-cd)](https://github.com/argoproj/argo-cd/releases/latest)
 [![Artifact HUB](https://img.shields.io/endpoint?url=https://artifacthub.io/badge/repository/argo-cd)](https://artifacthub.io/packages/helm/argo/argo-cd)
+[![SLSA 3](https://slsa.dev/images/gh-badge-level3.svg)](https://slsa.dev)
 
 **Code:** 
 [![Integration tests](https://github.com/argoproj/argo-cd/workflows/Integration%20tests/badge.svg?branch=master)](https://github.com/argoproj/argo-cd/actions?query=workflow%3A%22Integration+tests%22)