add spritesheet

assets/sokoban/License.txt

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+	
+
+	Sokoban (pack)
+
+	by  Kenney Vleugels (Kenney.nl)
+
+			------------------------------
+
+	License (Creative Commons Zero, CC0)
+	http://creativecommons.org/publicdomain/zero/1.0/
+
+	You may use these assets in personal and commercial projects.
+	Credit (Kenney or www.kenney.nl) would be nice but is not mandatory.
+
+			------------------------------
+
+	Donate:   http://support.kenney.nl
+	Request:  http://request.kenney.nl
+
+	Follow on Twitter for updates:
+	@KenneyNL