basic mcl_damage API

mods/CORE/mcl_damage/API.md

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+# mcl_damage
+This mod provide damage handling.
+
+## mcl_damage.register_modifier(func, priority)
+Register damage modifier.
+* func: function, called with (obj, damage, reason)
+
+    This function can modify damage, based on mcl reason.
+
+* priority: int, define call order of registered functions
+  
+    You should make use higher values for important or most used functions.