Added openssf badge to README.md
<!-- join kubernetes slack channel for cluster-api-aws-provider -->
<a href="http://slack.k8s.io/">
<img src="https://img.shields.io/badge/join%20slack-%23cluster--api--aws-brightgreen"></a>
+<!-- openssf badge -->
+<a href="https://bestpractices.coreinfrastructure.org/projects/5688">
+<img src="https://bestpractices.coreinfrastructure.org/projects/5688/badge"></a>
</p>
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