Add license for Go gopher

Author: ValorZard

ebiten-game/LICENSE.md

@@ -0,0 +1,7 @@
+## game/gopher.png
+
+```
+The Go gopher was designed by Renee French. (http://reneefrench.blogspot.com/)
+The design is licensed under the Creative Commons 4.0 Attributions license.
+Read this article for more details: https://blog.golang.org/gopher
+```