Add makefile and improve error message for no args

Makefile

@@ -0,0 +1,7 @@
+GOBUILD ?= go build
+
+all: main.go logging.go keyExchange.go keyCrypto.go files.go fileCrypto.go deviceDiscovery.go config.go
+	$(GOBUILD)
+
+install: opensend
+	install -Dm755 opensend $(DESTDIR)/usr/bin/opensend

main.go

@@ -176,6 +176,7 @@ func main() {
 		// Execute JSON action using files within opensend directory
 		config.ExecuteAction(opensendDir)
 	} else {
+		flag.Usage()
 		log.Fatal().Msg("You must choose sender or receiver mode using -s or -r")
 	}
 	// Remove opensend directory