Add documentation on how to integrate gamemode in a game.

[terraquad]

Is your feature request related to a problem? Please describe.
I am making a Zig game with OpenGL and I would like to make my game request gamemode on startup (like Mach Gamemode). The thing is, I don't want to use Mach libraries since they require me to downgrade my zig version. Now, I'm struggling on how to implement gamemode requests myself, since I couldn't find any documentation on how to do it.

Describe the solution you'd like
Some kind of documentation that describes the process on how to request gamemode from within a game.