Add API documentation for mcl_jukeboxe

mods/ITEMS/mcl_jukebox/API.md

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+# mcl_jukebox
+
+## mcl_jukebox.register_record(title, author, identifier, image, sound)
+
+* title: title of the track
+* author: author of the track
+* identifier: short string used in the item registration
+* image: the texture of the track
+* sound: sound file of the track
+
+## mcl_jukebox.registered_records
+
+Table indexed by item name containing:
+* title: title of the track
+* author: author of the track
+* identifier: short string used in the item registration
+* image: the texture of the track
+* sound: sound file of the track