Add examples

[balroggg]

Fix #42

[Xuanwo]

Thanks a lot!

[balroggg]

Maybe useful to add example with closure.
Something like

let f = || async {
    //
    Err::<(), anyhow::Error>(anyhow::anyhow!("not retryable"))
};
let backoff = todo!();
f.retry(&backoff).await();

[Xuanwo]

Maybe useful to add example with closure.

LGTM! This PR is good enough to get merged now. Do you want to add it in this PR or start a new one?

[balroggg]

Maybe useful to add example with closure.

LGTM! This PR is good enough to get merged now. Do you want to add it in this PR or start a new one?

Nice! I think in new one.