Add an hint to get pressure plates working better

mechanisms.lua

--- Thanks to sofar for helping with that code.
+--[[ Thanks to sofar for helping with that code.
+Pressure plates work better with this setting in minetest.conf (requires 0.4.14):
+	nodetimer_interval = 0.1
+]]
+
 local plate = {}
 screwdriver = screwdriver or {}