docs(readme): add OpenSSF Scorecard badge

The scorecard workflow already publishes results (publish_results: true);
this surfaces the score in the README badge row.

Badge image: api.securityscorecards.dev; click-through: scorecard.dev viewer.

Author: fgogolli

README.md

@@ -26,6 +26,7 @@
   <a href="https://github.com/finos/open-resource-broker/actions/workflows/security-code.yml"><img src="https://github.com/finos/open-resource-broker/actions/workflows/security-code.yml/badge.svg" alt="Security Scanning"></a>
   <a href="https://codecov.io/gh/finos/open-resource-broker"><img src="https://codecov.io/gh/finos/open-resource-broker/graph/badge.svg" alt="Coverage"></a>
   <a href="https://github.com/finos/open-resource-broker/actions/workflows/docs.yml"><img src="https://github.com/finos/open-resource-broker/actions/workflows/docs.yml/badge.svg" alt="Documentation"></a>
+  <a href="https://scorecard.dev/viewer/?uri=github.com/finos/open-resource-broker"><img src="https://api.securityscorecards.dev/projects/github.com/finos/open-resource-broker/badge" alt="OpenSSF Scorecard"></a>
 </p>
 
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