describe how to trigger perf runs on PRs

src/profiling.md

@@ -5,7 +5,10 @@ This section talks about how to profile the compiler and find out where it spend
 Depending on what you're trying to measure, there are several different approaches:
 
 - If you want to see if a PR improves or regresses compiler performance:
-  - The [rustc-perf](https://github.com/rust-lang/rustc-perf) project makes this easy and can be triggered to run on a PR via the `@rustc-perf` bot.
+  - The [rustc-perf](https://github.com/rust-lang/rustc-perf) project makes this easy and can be triggered to run on a PR via the `@rust-timer` bot.
+    The `@bors try @rust-timer queue` command, in a comment on the PR, will queue a try build and a
+    benchmarking run.
+    Note: you need `try` privileges to be able to do this. More details are available in the [perf collector documentation](https://github.com/rust-lang/rustc-perf/blob/master/collector/README.md).
 
 - If you want a medium-to-high level overview of where `rustc` is spending its time:
   - The `-Z self-profile` flag and [measureme](https://github.com/rust-lang/measureme) tools offer a query-based approach to profiling.