Add an example of using craterbot to run crater on regressions from the previous run

[petrochenkov]

Here (rust-lang/rust#73345 (comment)) are my attempts to infer the correct invocation from the docs (https://github.com/rust-lang/crater/blob/master/docs/bot-usage.md#available-crate-selections) and source code.

None of them worked until I found the PR implementing the feature (#461), and then followed some GitHub cross-links to find an actual example of use - rust-lang/rust#63809 (comment).

In particular, list:URL mentioned in the documentation doesn't work.