chore: add missing export step to README

Author: Bentroen

README.md

@@ -35,6 +35,7 @@ from nbswave import *
 render_audio("song.nbs", "output", format='wav')
 ```
 
+> [!NOTE]
 > Compatibility with audio formats depends on your FFmpeg configuration.
 
 ### Custom instruments
@@ -72,9 +73,14 @@ renderer.load_instruments("some_more_instruments.zip")
 
 renderer.missing_instruments()
 
-renderer.mix_song()
+track = renderer.mix_song()
+
+track.save("song.mp3")
 ```
 
+> [!TIP]
+> For additional parameters, check out the source code!
+
 ## Contributing
 
 Contributions are welcome! Make sure to open an issue discussing the problem or feature suggestion before creating a pull request.