Hi! You need to write it manually.
For all emulators:
/home/pi/RetroPie/configs/all/retroarch.cfg
If you want that changes only affect a single emulator edit retroarch.cfg in his own directory, and copy the lines from the “all” file:
/home/pi/RetroPie/configs/snes/retroarch.cfg
/home/pi/RetroPie/configs/mame/retroarch.cfg
