Add API documentation for screwdriver mod

mods/ITEMS/screwdriver/API.md

@@ -0,0 +1,20 @@
+Screwdriver API
+---------------
+
+The screwdriver API allows you to control a node's behaviour when a screwdriver is used on it.
+NOTE: This API is compatible with Minetest Game 5.1.0, but has some extensions.
+
+To use it, add the `on_rotate` function to the node definition.
+
+`on_rotate(pos, node, user, mode, new_param2)`
+
+ * `pos`: Position of the node that the screwdriver is being used on
+ * `node`: that node
+ * `user`: The player who used the screwdriver
+ * `mode`: `screwdriver.ROTATE_FACE` or `screwdriver.ROTATE_AXIS`
+ * `new_param2` the new value of `param2` that would have been set if `on_rotate` wasn't there
+ * return value: false to disallow rotation, nil to keep default behaviour, true to allow
+ 	it but to indicate that changed have already been made (so the screwdriver will wear out)
+ * use `on_rotate = false` to always disallow rotation
+ * use `on_rotate = screwdriver.rotate_simple` to allow only face rotation
+ * use `on_rotate = screwdriver.rotate_3way` (MineClone 2 extension) for pillar-like nodes that should only have 3 possible orientations)