Expand message that shows when Interface exists because another instance is already running.

interface/src/main.cpp

@@ -500,6 +500,7 @@ int main(int argc, const char* argv[]) {
             socket.close();
 
             qDebug() << "Interface instance appears to be running, exiting";
+            qDebug() << "Start with --allowMultipleInstances to allow running multiple instances at once.";
 
             return EXIT_SUCCESS;
         }