Fix Readme

[qui-jason]

Example code doesn't verify key custody/chaining.

This should be added:

            if serder.est()? {
                let digers = if pserder.est()? {
                    pserder.digers()?
                } else {
                    let (event, _) = self.store.get_latest_establishment_event_as_of_sn(&pre, sn as u32 - 1)?;
                    Serder::new_with_raw(event.as_bytes())?.digers()?
                };

                for (i, diger) in digers.iter().enumerate() {
                    if !diger.verify(&verfers[i].qb64b()?)? {
                        return err!(Error::Verification);
                    }
                }
            }

[qui-jason]

The sn as u32 cast is only a limitation of the underlying datastore, it should be a u128. We have some time before 4 billion events occur I would think, but this is something to be aware of.