Add warning message about game ID detection

README.md

@@ -193,3 +193,6 @@ options. Options specified on the command line will override those in
 System3-sdl2 supports localization, whereas the original System1-3 only
 supported Japanese. If you're interested in translating games, check out
 [Sys0Decompiler](https://alicesoft.miraheze.org/wiki/User_blog:RottenBlock/System_Programming_Resources).
+
+When running a modified (translated) game, system3 cannot automatically detect
+the game ID. You need to specify the `game` option in `system3.ini`.

src/sdlmain.cpp

@@ -82,6 +82,12 @@ int main(int argc, char *argv[])
 #endif
 	} else {
 		WARNING("Cannot determine game id. crc32_a: %08x, crc32_b: %08x", g_nact->crc32_a, g_nact->crc32_b);
+		SDL_ShowSimpleMessageBox(
+			SDL_MESSAGEBOX_WARNING, "system3",
+			"Unable to determine game ID.\n"
+			"If you are running a modified game, plsese specify 'game = <original-game-id>' in system3.ini.\n"
+			"See README.md for more information.",
+			g_window);
 	}
 
 #ifdef __EMSCRIPTEN__