Add badge for SLSA Level 3

Signed-off-by: Matheus Pimenta <matheuscscp@gmail.com>

README.md

@@ -5,6 +5,7 @@
 [![OpenSSF Scorecard](https://api.securityscorecards.dev/projects/github.com/fluxcd/flux2/badge)](https://api.securityscorecards.dev/projects/github.com/fluxcd/flux2)
 [![FOSSA Status](https://app.fossa.com/api/projects/custom%2B162%2Fgithub.com%2Ffluxcd%2Fflux2.svg?type=shield)](https://app.fossa.com/projects/custom%2B162%2Fgithub.com%2Ffluxcd%2Fflux2?ref=badge_shield)
 [![Artifact HUB](https://img.shields.io/endpoint?url=https://artifacthub.io/badge/repository/flux2)](https://artifacthub.io/packages/helm/fluxcd-community/flux2)
+[![SLSA 3](https://slsa.dev/images/gh-badge-level3.svg)](https://fluxcd.io/flux/security/slsa-assessment)
 
 Flux is a tool for keeping Kubernetes clusters in sync with sources of
 configuration (like Git repositories and OCI artifacts),