Recommend Flags in README

[yuuwe-n]

Earlier I had issues with drivers, and I believe that if we recommend to enable USE flags for mesa like vulkan and opencl, that less people will have issues.

I was recommended to create an issue after solving my problem: https://www.reddit.com/r/Gentoo/comments/on6auq/very_slow_performance_on_games_with_steam_using/h5pzhxw?utm_source=share&utm_medium=web2x&context=3

If you want I can do a pull request and add instructions or some troubleshooting steps to the readme