Link to, or write a blog post about, the content of etcd/contrib/lock/README.md

Consider writing a blog entry to include all or part of the content of https://github.com/etcd-io/etcd/blob/master/contrib/lock/README.md. Or, at a minimum, add a few sentences to **[Notes on the usage of lock and lease][LnL]**.

- Followup to #188
- Context: https://github.com/etcd-io/website/issues/188#issuecomment-814354984

[LnL]: https://etcd.io/docs/next/learning/why/#notes-on-the-usage-of-lock-and-lease