Added a join slack badge in CAPA repo for k8s cluster-api-aws channel
<!-- goreportcard badge -->
<a href="https://goreportcard.com/report/sigs.k8s.io/cluster-api-provider-aws">
<img src="https://goreportcard.com/badge/sigs.k8s.io/cluster-api-provider-aws"></a>
+<!-- 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>
</p>
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