no-panic currently cannot be used in doctests

Attempting to use no-panic in a library requires the library to be used somehow, somewhere. The obvious answer to that is tests. For that, tests need to be compiled with `opt-level=1` (as per README's dev profile). However, when the tests are run, it seems that while the regular integration tests _do_ pass, the doctests fail with a linker error.

This is an artifact of https://github.com/rust-lang/cargo/issues/4251

I think no-panic README should point this out at least, but IMO ideally it would also not even suggest to tweak the dev-profile but _only_ suggest the optional dependency solution, as everything else seems to be a dead end.