refactor mod (#22)

doc/gameid.md

@@ -0,0 +1,12 @@
+# GameId API
+
+## minetest versions >= 5.7 
+
+simply returns `minetest.get_game_info().id`  
+
+## minetest versions < 5.7
+
+approximates the gameid value via a hardcoded table of gameid=>modname 
+and then checks via `minetest.get_modpath()`. If it fails, it falls 
+back to using `xcompat_unknown_gameid` as the id. See the chart in the
+readme for which games are supported