Add a bit about crater

src/tests/intro.md

@@ -170,7 +170,63 @@ communicate with the server to coordinate running tests (see
 
 ## Crater
 
-TODO
+[Crater](https://github.com/rust-lang-nursery/crater) is a tool for compiling
+and running tests for _every_ crate on [crates.io](https://crates.io/) (and a
+few on GitHub). It is mainly used for checking for extent of breakage when
+implementing potentially breaking changes and ensuring lack of breakage by
+running beta vs stable compiler versions.
+
+### When to run Crater
+
+You should request a crater run if your PR makes large changes to the compiler
+or could cause breakage. If you are unsure, feel free to ask your PR's reviewer.
+
+### Requesting Crater Runs
+
+The rust team maintains a few machines that can be used for running crater runs
+on the changes introduced by a PR. If your PR needs a crater run, leave a
+comment for the triage team in the PR thread. Please inform the team whether
+you require a "check-only" crater run, a "build only" crater run, or a
+"build-and-test" crater run. The difference is primarily in time; the
+conservative (if you're not sure) option is to go for the build-and-test run.
+If making changes that will only have an effect at compile-time (e.g.,
+implementing a new trait) then you only need a check run.
+
+Your PR will be enqueued by the triage team and the results will be posted when
+they are ready. Check runs will take around ~3-4 days, with the other two
+taking 5-6 days on average.
+
+While crater is really useful, it is also important to be aware of a few
+caveats:
+
+- Not all code is on crates.io! There is a lot of code in repos on GitHub and
+  elsewhere. Also, companies may not wish to publish their code. Thus, a
+  successful crater run is not a magically green light that there will be no
+  breakage; you still need to be careful.
+
+- Crater only runs Linux builds on x86_64. Thus, other architectures and
+  platforms are not tested. Critically, this includes Windows.
+
+- Many crates are not tested. This could be for a lot of reasons, including
+  that the crate doesn't compile any more (e.g. used old nightly features),
+  has broken or flaky tests, requires network access, or other reasons.
+
+- Before crater can be run, `@bors try` needs to suceed. This means that if
+  your code doesn't compile or fails tests, you cannot run crater.
+
+## Perf runs
+
+A lot of work is put into improving the performance of the compiler and
+preventing performance regressions. A "perf run" is used to compare the
+performance of the compiler in different configurations for a large collection
+of popular crates. Different configurations include "fresh builds", builds
+with incremental compilation, etc.
+
+The result of a perf run is a comparison between two versions of the
+compiler (by their commit hashes).
+
+You should request a perf run if your PR may affect performance, especially
+if it can affect performance adversely.
 
 ## Further reading