mentioning opt-level in the readme

[Zizico2]

I had a code-base where this was the test profile:

[profile.test]
opt-level = 3

I spent more time then I'd like to admit figuring out why, in unit tests faux was behaving correctly but in integration tests I was getting `SignaturIt::create_signature<E,D>` was called but never stubbed. As you can see the function is generic maybe that's part of the problem. I am mocking it only for a specific type. Removing opt-level = 3 fixed it.

Maybe this is worth mentioning somewhere

[nrxus]

Hm that is interesting. opt-level = 3 is the default when building with release and we have a test release step in CI that's been passing and all tests also pass in my machine on release mode.

Could you provide a minimized example that'd fail on release but pass on debug mode?