Add a guide on how to use Rivatuner on TF2

[Leololgithub]

Is your feature request related to a problem? Please describe.
Well, You said in mastercomfig docs that using rivatuner is more recommended since tf2's frame limiter is kinda inconsistent.

Describe the solution you'd like
I would like it if you would explain how to setup Rivatuner for TF2 since i tried doing it myself and i couldn't get it to work.

Thank You and have a good day.

[AmyRoxwell]

IIRC, the equivalent of rivatunner on linux is mangohud, it has a limiter and everything