Add API documentation

mods/HUD/mcl_info/API.md

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+## mcl_info
+An api to make custom entries in the mcl2 debug hud.
+
+### mcl_info.register_debug_field(name,defintion)
+Debug field defintion example:
+{
+	level = 3,
+	--show with debug level 3 and upwards
+
+	func = function(player,pos) return minetest.pos_to_string(pos) end,
+	-- Function that is run for at each debug
+	-- sample (default: every .63 seconds)
+	-- It should output a string and determines
+	-- the content of the debug field.
+}
+
+### mcl_info.registered_debug_fields
+Table the debug definitions are stored in. Do not modify this directly. If you need to overwrite a field just set it again with mcl_info.register_debug_field().