add digilines doc file and document switching station interface

partially attends to #53

README.md

@@ -48,6 +48,7 @@ Ingame:
 * [Powered machines](./technic/doc/machines.md)
 * [Generators](./technic/doc/generators.md)
 * [Forceload anchor](./technic/doc/anchor.md)
+* [Digilines](./technic/doc/digilines.md)
 
 Mod development:
 * [Api](./technic/doc/api.md)

technic/doc/digilines.md

@@ -0,0 +1,32 @@
+
+# Technic digilines compatibility
+
+This document describes the interaction of
+technic machines with the `digilines` mod (https://github.com/minetest-mods/digilines)
+
+## Switching station
+
+**NOTE**: make sure the channel is set accordingly, "switch" in this case
+
+Get power and lag stats:
+```lua
+if event.type == "program" then
+	-- start interrupt cycle
+	interrupt(1)
+end
+
+if event.type == "interrupt" then
+	-- request stats
+	digiline_send("switch", "GET")
+end
+
+if event.type == "digiline" and event.channel == "switch" then
+	--[[
+	event.msg = {
+		demand = 0, -- demand in EU's
+		supply = 0, -- supply in EU's
+		lag = 0 -- generated lag in microseconds
+	}
+	--]]
+end
+```