Move monster spawner mod to ITEMS

mods/ITEMS/mcl_monster_spawner/README.md

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+This mod adds a monster spawner for MineClone 2.
+Monsters will appear around the monster spawner in semi-regular intervals.
+
+This mod is originally based on the mob spawner from Mobs Redo by TenPlus1
+but has been modified quite a lot to fit the needs of MineClone 2.
+
+Players can get a monster spawner by `giveme` and is initially empty after
+placing.
+
+## Programmer notes
+To set the mob spawned by a monster spawner, first place the monster spawner
+(e.g. with `minetest.set_node`), then use the function
+`mcl_monster_spawner.setup_spawner` to set its attributes. See the comment
+in `init.lua` for more info.
+
+## License (code and texture)
+MIT License