Please add a LICENSE file to this repo

[gmlewis]

Thank you, Brendan, for your Designing Distributed Systems ebook and for this code repo!

If you could please add a top-level LICENSE file to this repo, it would be greatly appreciated and would enable other individuals, teams, and organizations to check out and learn from your repo.

Obviously, it is up to you and/or O'Reilly as to which license you choose, but ideally it would be something like Apache-2.0, MIT, or BSD.