FAQ: Further env var required for WayFire headless

[headbank]

Just a note that the directions given in FAQ.md did not suffice for starting a headless WayFire session. I also had to set this additional variable:
export WLR_RENDER_DRM_DEVICE=/dev/dri/renderD128

wayfire-session does emit an error about this variable being unset, but I had to comb around quite a bit before learning the value I needed to set.

I'm not sure whether the difference with Sway is because Sway doesn't unconditionally require DRM as WayFire does, or simply that Sway takes care of setting the variable internally, but mentioning this extra requirement in the headless FAQ might save other newcomers a bit of time and confusion.

[Or maybe it's Wayfire's responsibility to avoid this issue; if so, let me know and I'll go bother them.]