Document how to run perf on PRs

[spastorino]

Would be nice to add @bors try @rust-timer queue somewhere in the guide.

The current place where we can find this is in https://github.com/rust-lang/rustc-perf/blob/master/collector/README.md?plain=1#L49 but I guess it would be nice to have it in the guide somewhere.

[camelid]

I also added a help page to rustc-perf a few months ago that has instructions: https://perf.rust-lang.org/help.html