This looks pretty cool, it doesn't explain how to use Let's Encrypt as far as I can find in the docs, have an example?

EDIT: Looks like it's available in the latest prerelease. https://github.com/mholt/caddy/releases

Yep, check back tomorrow. Meanwhile here's a 28-second teaser video of it working: https://www.youtube.com/watch?v=nk4EWHvvZtI