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
This is what I meant was false. We definitely wait at least [60 seconds](https://github.com/progrium/dokku/blob/master/dokku#L111) - configurable - before killing old containers.