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
There's a short wait for the container to start, then the checks are run against it, before switching traffic. So if the new container is not able to handle request, traffic keeps flowing to the old one, new container gets destroyed.

The long timeout servers two purposes. First, some responses are just slow. Second long lasting connections (eg server sent events), helps to gradually migrate those over to new server.