added svcat plugin (#7452)

Including a Kubernetes Service Catalog plugin 

plugins/svcat/README.md

+# svcat
+
+This plugin provides completion for the [Kubernetes service catalog cli](https://github.com/kubernetes-incubator/service-catalog).
+
+To use it, add `svcat` to the plugins array in your zshrc file.
+
+```
+plugins=(... svcat)
+```

plugins/svcat/svcat.plugin.zsh

+# Autocompletion for svcat.
+#
+
+if [ $commands[svcat] ]; then
+  source <(svcat completion zsh)
+fi