added todo.md

todo.md

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+# Todo list
+
+    * CSS
+    * More levels
+    * Ending after last level
+    * Scoreboard
+    
+## CSS
+
+    * Navigation bar
+    
+## More levels
+
+    * https://alliballibaba.github.io/Sokoban-Level-Generator/
+
+## Scoreboard
+
+    * Prompt for username after ending
+    * Table with filled usernames