wip mcl_damage documentation

mods/CORE/mcl_damage/API.md

@@ -0,0 +1,15 @@
+# mcl_damage
+
+This mod is intended to overall minetest's native damage system, to provide a better integration between features that deals with entities' health.
+
+WARNING: Not using it inside your mods may cause strange bugs (using the native damage system may cause conflicts with this system).
+
+## Callbacks
+
+To modify the amount of damage made by something:
+
+```lua
+--obj: an ObjectRef
+mcl_damage.register_modifier(function(obj, damage, reason)
+end, 0)
+```