Document that the user shouldn't set up their own panic hook

The fuzzer has its own panic hook, which is useful when
not using `-C panic=abort`.

README.md

@@ -175,6 +175,7 @@ For instance:
 - Never ever call `std::process::exit()`.
 - Disable logging and other unnecessary functionnalities.
 - Try to avoid modifying global state when possible.
+- Do not set up your own panic hook when run with `cfg(fuzzing)`
 
 
 When building with `cargo hfuzz`, the argument `--cfg fuzzing` is passed to `rustc` to allow you to condition the compilation of thoses adaptations thanks to the `cfg` macro like so: