Add info on how to iterate over existing buckets

[ErikKalkoken]

This adds a short description on how to iterate over existing buckets to the README.

This should be helpful for people looking for this particular information (e.g. see #327).

[ahrtr]

Thanks for the PR.

Please signoff the commit. Read https://github.com/etcd-io/bbolt/pull/828/checks?check_run_id=29221691728

[ivanvc]

/ok-to-test

[ivanvc]

Hi @ErikKalkoken, thanks for your contribution. Could you please sign your commit, as requested by Ben? So the developer certificate of origin (DCO) check passes, i.e:

git rebase HEAD~1 --signoff
git push --force

Thanks!

Please refer to: https://github.com/etcd-io/etcd/blob/main/CONTRIBUTING.md#commit-your-change

[ahrtr]

LGTM

Thanks.

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

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