Add docs for CHECKS

Fixes #693

The docs are based on #562 and @assaf's [blog post](https://labnotes.org/zero-downtime-deploy-with-dokku/).

### Review

@josegonzalez @michaelshobbs
cc @travischoma
Maybe my explanation wasn't clear. Although we keep the old container around, new traffic is sent to the new container right away.

This can lead to 502s if your app doesn't bind to the port right away, an easy way to repro this is using a setTimeout around the listen call in Heroku's sample node.js app.

Maybe this is an edge-case and not worth bringing up in the docs.

Thoughts?