Add messages to indicate that opening an external firewall may be necessary.

pkg/kubectl/cmd/expose.go

@@ -201,5 +201,15 @@ func RunExpose(f *cmdutil.Factory, out io.Writer, cmd *cobra.Command, args []str
 		}
 	}
 
+	if cmdutil.GetFlagBool(cmd, "create-external-load-balancer") {
+		msg := fmt.Sprintf(`
+			An external load-balanced service was created.  On many platforms (e.g. Google Compute Engine),
+			you will also need to explicitly open a firewall rule for the service port (%d) to serve traffic.
+			
+			See https://github.com/GoogleCloudPlatform/kubernetes/tree/master/docs/services-firewall.md for more details.
+			`, cmdutil.GetFlagInt(cmd, "port"))
+		out.Write([]byte(msg))
+	}
+
 	return f.PrintObject(cmd, object, out)
 }