Adding useful commands to documentation

[tiif]

I used the commands below when enabling more tests in Tokio. We can consider adding these commands to README as they could be useful to others as well.

1. To run the entire test suite without terminating on unsupported FFI error, cargo miri nextest run -- no-fail-fast can be used.
2. If the unsupported FFI errors occur in docs test, adding# if cfg!(miri) { return } will allow Miri to skip the test while still executing it normally with cargo test.

[RalfJung]

That doesn't really fit in the existing README, it'd be a new section on "how to use Miri on a crate that has many unsupported parts". I think our README will collapse from feature creep if we keep adding stuff like that to it, so IMO this requires us to first transition to a multi-file structure.

[tiif]

@rustbot label +A-docs +C-enhancement