Add instructions on how to use it on updated CSS/HL2DM/TF2/DoD:S

[Jaffies]

Since https://www.supergoodcode.com/the-finale/ got up. There was a way to compile Xnine and get sdl_xnine.so, that could have been enabled via LD_PRELOAD.
This can be done with Portal 2, L4D2, and CS:GO
But this can't be done with new updated mp games which now support dxvk too.