warn about custom rules

OpenRA.Game/Server/Server.cs

@@ -374,6 +374,9 @@ namespace OpenRA.Server
 						SendOrderTo(newConn, "Message", motd);
 				}
 
+				if (Map.RuleDefinitions.Any() && !LobbyInfo.IsSinglePlayer)
+					SendOrderTo(newConn, "Message", "This map contains custom rules. Game experience may change.");
+
 				if (handshake.Mod == "{DEV_VERSION}")
 					SendMessage("{0} is running an unversioned development build, ".F(client.Name) +
 					"and may desynchronize the game state if they have incompatible rules.");

OpenRA.Mods.Common/ServerTraits/LobbyCommands.cs

@@ -371,6 +371,9 @@ namespace OpenRA.Mods.Common.Server
 
 						server.SendMessage("{0} changed the map to {1}.".F(client.Name, server.Map.Title));
 
+						if (server.Map.RuleDefinitions.Any())
+							server.SendMessage("This map contains custom rules. Game experience may change.");
+
 						return true;
 					}
 				},