Add OpenSSF best practices badge

Signed-off-by: Kate Goldenring <kate.goldenring@fermyon.com>

Author: kate-goldenring

README.md

@@ -6,6 +6,7 @@
   <p>Spin is a framework for building, deploying, and running fast, secure, and composable cloud microservices with WebAssembly.</p>
       <a href="https://github.com/fermyon/spin/actions/workflows/build.yml"><img src="https://github.com/fermyon/spin/actions/workflows/build.yml/badge.svg" alt="build status" /></a>
       <a href="https://cloud-native.slack.com/archives/C089NJ9G1V0"><img alt="Slack" src="https://img.shields.io/badge/slack-spin-green.svg?logo=slack"></a>
+      <a href="https://www.bestpractices.dev/projects/10373"><img src="https://www.bestpractices.dev/projects/10373/badge"></a>
 </div>
 
 ## What is Spin?