Updated RGUI (markdown)

RGUI.md

@@ -180,4 +180,15 @@ Input configs have a simple format, e.g.:
 Since binds are per-device and not per-player, input_playerN_* prefixes are changed to just input_*.
 
 ## PlayStation disk swapping
-TODO
+Some PlayStation games require disk swapping. To manage disks, libretro has a concept of a disk index (like a multi-disk CD player, remember those? :P).
+If a PlayStation core is loaded, `Disk Index` and `Disk Image Append` options are added to main menu.
+
+In Mednafen PSX, multi-CD images are typically handled with an .m3u playlist. In this case, you can swap disks by cycling through the Disk Index setting. If you don't or can't use a playlist, you can append a disk image on the fly using `Disk Image Append`. In this case, you browse for a disk image and append it to the internal disk image list. The `Disk Index` is updated appropriately and you are returned to the game.
+
+### Workarounds
+Replacing the disk inside RGUI is "physically" speaking the same as ejecting, swapping disks and closing the tray instantanously. Some games will not work with this approach, notably Metal Gear Solid because they need to detect an actual eject taking place.
+
+To work around this, set `Disk Index` to `No Disk`, and exit RGUI. The game will pick up that the tray has been ejected/missing disk after half a second or so. Now you can go back to RGUI, pick the correct disk index and return to the game.
+
+### Note
+This feature is still fairly new, and might change behavior over time as kinks (as noted above) are worked out.