chore: update music text file

board/recalbox/fsoverlay/recalbox/share_init/music/readme.txt

-Place your mp3 or ogg files here to replace default bg music in emulationstation.
-The files must have a sampling rate of 44100Hz and shouldn't have a higher bit rate than 256 kb/s.
+Place your music files here to replace default background music in EmulationStation.
+Supported formats:
+- OGG (*.ogg)
+- OGG OPUS (*.opus)
+- MP3 (*.mp3)
+- FLAC (*.flac)
+- WAVE (*.wav)
+- Modules (*.mod *.s3m *.xm *.it *.669 *.apun *.dsm *.far *.amf *.gdm *.imf *.med *.mtm *.okt *.stm *.stx *.ult *.uni)
+- MIDI files (*.mid *.midi)
+
+Beware: For MIDI file, there is *NO* default soundfont installed.
+You must install a soundfont (named soundfont.sf2) in this folder.
+Some websites where to download soundfonts:
+- Generic: http://timtechsoftware.com/uploads/GeneralUser_GS_SoftSynth_v144.sf2
+- Minmalist: https://musical-artifacts.com/artifacts/713
+- Collections & game-related soundfonts: https://musical-artifacts.com/artifacts?formats=sf2