Update application usage

main.go

@@ -3,6 +3,7 @@ package main
 import (
 	"flag"
 	"fmt"
+	"os"
 
 	"github.com/mtojek/aws-closest-region/closest"
 	log "github.com/sirupsen/logrus"
@@ -11,6 +12,7 @@ import (
 func main() {
 	var verbose bool
 	flag.BoolVar(&verbose, "verbose", false, "verbose mode")
+	flag.Usage = usage
 	flag.Parse()
 	log.SetFormatter(&log.TextFormatter{
 		DisableLevelTruncation: true,
@@ -38,3 +40,8 @@ func main() {
 
 	fmt.Println(closestEndpoint)
 }
+
+func usage() {
+	fmt.Fprintf(flag.CommandLine.Output(), "Usage of %s <flags> [serviceName]:\n", os.Args[0])
+	flag.PrintDefaults()
+}