Extend comment to explain why we shouldn't join the thread for the panic example.

Hi,
Can we extend [this comment here](https://github.com/RoxasShadow/rollbar-rs/blob/master/examples/panic.rs#L18) on not joining the thread in the panic handler with an explanation as to why? without joining, I worry that any panic crashing my program will be lost and not submitted to rollbar (which is what happens in the example given).