add SLSA badge to Readme

Author: eliheady

README.md

@@ -3,6 +3,7 @@
 [![StackExchange/dnscontrol/build](https://github.com/StackExchange/dnscontrol/actions/workflows/pr_build.yml/badge.svg)](https://github.com/StackExchange/dnscontrol/actions/workflows/pr_build.yml)
 [![Google Group](https://img.shields.io/badge/google%20group-chat-green.svg)](https://groups.google.com/forum/#!forum/dnscontrol-discuss)
 [![PkgGoDev](https://pkg.go.dev/badge/github.com/StackExchange/dnscontrol)](https://pkg.go.dev/github.com/StackExchange/dnscontrol/v4)
+[![SLSA 3](https://slsa.dev/images/gh-badge-level3.svg)](https://slsa.dev)
 
 [DNSControl](https://docs.dnscontrol.org/) is a system
 for maintaining DNS zones.  It has two parts: