Add doctl autocompletion (#6501)

plugins/doctl/doctl.plugin.zsh

@@ -0,0 +1,9 @@
+# Autocompletion for doctl, the command line tool for DigitalOcean service
+#
+# doctl project: https://github.com/digitalocean/doctl
+#
+# Author: https://github.com/HalisCz
+
+if [ $commands[doctl] ]; then
+  source <(doctl completion zsh)
+fi