N3DS: Document the SDL_GetBasePath behaviour.

docs/README-n3ds.md

@@ -24,3 +24,4 @@ cmake --install build
 -   Currently only software rendering is supported.
 -   SDL2main should be used to ensure ROMFS is enabled.
 -   By default, the extra L2 cache and higher clock speeds of the New 2/3DS lineup are enabled. If you wish to turn it off, use `osSetSpeedupEnable(false)` in your main function.
+-   `SDL_GetBasePath` returns the romfs root instead of the executable's directory.