document shunting mode, add unset_shunt to luaatc

advtrains_luaautomation/README.txt

@@ -203,6 +203,10 @@ set_rc(routingcode)
 split_at_index(index, command)
 	Splits the train at the specified index, into a train with index-1 wagons and a second train starting with the index-th wagon.
 	command specifies an atc command to be sent to the second train after decoupling. 
+set_shunt()
+	Sets the train into shunting mode
+unset_shunt()
+	Sets the train into normal mode
 
 # Operator panel
 This simple node executes its actions when punched. It can be used to change a switch and update the corresponding signals or similar applications.

advtrains_luaautomation/atc_rail.lua

@@ -71,6 +71,10 @@ function r.fire_event(pos, evtdata)
 			if not train_id then return false end
 			train.is_shunt = true
 		end,
+		unset_shunt = function()
+			if not train_id then return false end
+			train.is_shunt = nil
+		end,
 		set_line = function(line)
 			if type(line)~="string" and type(line)~="number" then
 				return false