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

[petrochenkov]

Here (https://github.com/rust-lang/rust/pull/73345#issuecomment-647126019) 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 (https://github.com/rust-lang/crater/pull/461), and then followed some GitHub cross-links to find an actual example of use - https://github.com/rust-lang/rust/pull/63809#issuecomment-541798332.

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