Update README.md

It wan't obvious for me how to join this channel.

README.md

@@ -4,6 +4,7 @@ Deploy a Production Ready Kubernetes Cluster
 ============================================
 
 If you have questions, join us on the [kubernetes slack](https://kubernetes.slack.com), channel **\#kubespray**.
+You can get your invite [here](http://slack.k8s.io/)
 
 -   Can be deployed on **AWS, GCE, Azure, OpenStack, vSphere, Oracle Cloud Infrastructure (Experimental), or Baremetal**
 -   **Highly available** cluster