update README to reflect the backoff duration accurately.

[le-cong]

currently we have '...with an exponential backoff of (2^retries * 100) milliseconds per retry with full jitter', which should be 2^(retries-1) * 100. we should update it and maybe give a comment example for the overall duration to make it easier to estimate.

[carlansley]

Update doc with examples when we implement #20