packer: document machine-readable

packer/environment.go

@@ -299,6 +299,9 @@ func (e *coreEnvironment) printHelp() {
 		// Output the command and the synopsis
 		e.ui.Say(fmt.Sprintf("    %v     %v", key, synopsis))
 	}
+
+	e.ui.Say("\nGlobally recognized options:")
+	e.ui.Say("    --machine-readable    Machine-readable output format.")
 }
 
 // Returns the UI for the environment. The UI is the interface that should