add API documentation to controls

mods/CORE/controls/API.md

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+# controls
+
+## controls.players
+Table containing player controls at runtime.
+WARNING: Never use this table in writing
+
+## controls.register_on_press(func)
+Register a function that will be executed with (player, keyname) every time a player press a key.
+
+## controls.registered_on_press
+Table containing functions registered with controls.register_on_press().
+
+## controls.register_on_release(func)
+Register a function that will be executed with (player, keyname, clock_from_last_press) every time a player release a key.
+
+## controls.registered_on_release
+Table containing functions registered with controls.register_on_release().
+
+## controls.register_on_hold(func)
+Register a function that will be executed with (player, keyname, clock_from_start_hold) every time a player hold a key.
+
+## controls.registered_on_hold
+Table containing functions registered with controls.register_on_hold().