Add documentation to mcl_wip (pretty useless...)

mods/MISC/mcl_wip/API.md

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+# mcl_wip
+Used to mark items or nodes as WIP.
+
+## mcl_wip.register_wip_item(itemname)
+Register <itemname> as a WIP item.
+If <itemname> isn't a valid itemname, an error will be shown after mods loaded.
+
+## mcl_wip.register_experimental_item(itemname)
+Register <itemname> as a experimental item.
+If <itemname> isn't a valid itemname, an error will be shown after mods loaded.
+
+## mcl_wip.registered_wip_items
+Table containing WIP items names.
+
+## mcl_wip.registered_experimental_items
+Table containing experimental items names.