add API.md for lightning mod

mods/ENVIRONMENT/lightning/API.md

@@ -0,0 +1,31 @@
+# lightning
+Lightning mod for MineClone2 with the following API:
+
+## lightning.register_on_strike(function(pos, pos2, objects))
+Custom function called when a lightning strikes.
+
+* `pos`: impact position
+* `pos2`: rounded node position where fire is placed
+* `objects`: table with ObjectRefs of all objects within a radius of 3.5 around pos2
+
+## lightning.strike(pos)
+Let a lightning strike.
+
+* pos: optional, if not given a random pos will be chosen
+* returns: bool - success if a strike happened
+
+
+### Examples:
+
+```
+lightning.register_on_strike(function(pos, pos2, objects)
+        for _, obj in pairs(objects) do
+                obj:remove()
+        end
+        minetest.add_entity(pos, "mobs_mc:sheep")
+end)
+
+minetest.register_on_respawnplayer(function(player)
+        lightning.strike(player:get_pos())
+end)
+```