add: string interface

interfaces/09-stringer/game.go

@@ -0,0 +1,14 @@
+// For more tutorials: https://blog.learngoprogramming.com
+//
+// Copyright © 2018 Inanc Gumus
+// Learn Go Programming Course
+// License: https://creativecommons.org/licenses/by-nc-sa/4.0/
+//
+
+package main
+
+type game struct {
+	// game satisfies the fmt.Stringer
+	// because the product satisfies the fmt.Stringer
+	product
+}