hint if people set collides on tilesets

lib/Properties.hs

@@ -197,6 +197,7 @@ checkTileset = do
     checkTilesetProperty :: Property -> LintWriter Tileset
     checkTilesetProperty p@(Property name _value) = case name of
       "tilesetCopyright" -> naiveEscapeProperty p
+      "collides"         -> warn "property \"collides\" should be set on individual tiles, not the tileset"
       _                  -> warn $ "unknown tileset property " <> prettyprint name
 
     checkTile :: Tile -> LintWriter Tileset