Document how to generate UI test .stderr files locally

[joshlf]

As uncovered in #183 and rust-lang/rust#116433, rustc's output - when run on our UI test files - differs depending on whether the rust-src component is installed for a given toolchain. As of #183, we require that component to be installed in CI, and so a developer testing or generating .stderr files locally must also have it installed. It doesn't always get installed by default, which can be a source of confusion (the resulting error is mysterious and gives no hints that rust-src is the culprit).

We should document this somewhere so that developers aren't confused.