Add command usage example

cmd/config/config.go

@@ -23,7 +23,7 @@ import (
 
 var Cmd = &cobra.Command{
 	Use:   "config",
-	Short: "Goma configuration",
+	Short: "Goma Gateway configuration management",
 	Run: func(cmd *cobra.Command, args []string) {
 		if len(args) == 0 {
 			return

cmd/root.go

@@ -28,7 +28,7 @@ var rootCmd = &cobra.Command{
 	Use:     "goma",
 	Short:   "Goma Gateway is a lightweight API Gateway, Reverse Proxy",
 	Long:    `.`,
-	Example: "",
+	Example: util.MainExample,
 	Version: util.FullVersion(),
 }