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
@josegonzalez what are you saying is false? The sentence regarding DOKKU_CHECKS_WAIT is true. We should also add a mention of DOKKU_WAIT_TO_RETIRE as that is was controls how long the old container has to fulfill in-flight requests before being killed.