docs: add SLSA level 3 badge

Author: fraxken

README.md

@@ -12,6 +12,9 @@
     <a href="https://api.securityscorecards.dev/projects/github.com/NodeSecure/js-x-ray">
       <img src="https://api.securityscorecards.dev/projects/github.com/NodeSecure/js-x-ray/badge?style=for-the-badge" alt="ossf scorecard">
     </a>
+    <a href="https://slsa.dev/spec/v1.0/levels#build-l3">
+      <img src="https://img.shields.io/badge/SLSA-level%203-green?style=for-the-badge&logo=data:image/png;base64,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" alt="slsa level3">
+    </a>
     <a href="https://github.com/NodeSecure/js-x-ray/actions?query=workflow%3A%22Node.js+CI%22">
       <img src="https://img.shields.io/github/actions/workflow/status/NodeSecure/js-x-ray/node.js.yml?style=for-the-badge" alt="github ci workflow">
     </a>