message: Add rust-timer bot.
diff: @@ -76,6 +76,12 @@ It is split into a data [collector] and a [web frontend]. The raw performance
  data is available [here](https://github.com/rust-lang-nursery/rustc-timing)
  and can be browsed on the [perf website](https://perf.rust-lang.org).
  +One-off performance runs can done by addressing the [rust-timer
  +bot](https://github.com/rust-timer). On a PR, do a "try" build (`@bors try`),
  +wait for it finish, then leave a comment with the merge commit hash like this:
  +
  + @rust-timer build ac89e0324b48993d6d148f598ea0e7357e229c79
  +
  [collector]: https://github.com/rust-lang-nursery/rustc-perf/tree/master/collector
  [web frontend]: https://github.com/rust-lang-nursery/rustc-perf/tree/master/site
  