Mention jkgfxmod installation in README

[Calinou]

This closes #188.

[shinyquagsire23]

So, OpenJKDF2 supports the mod end of jkgfxmod (everything in the jkgm/ folder except jkgm/shaders/), but not jkgm.json, renderer.dll, nor inject.exe. So there really isn't an install process per-se.

Though that said, I'm probably overdue for some better documentation on like, things that have been added to OpenJKDF2. Like for example, I specifically avoided listing out possible paths for resource folders on Linux because the ~official method is going to Expansion & Mods and click Open Resource Folder, avoids a lot of confusion and works on every platform.