Write guide for hooking up a deploy via Continuous Integration

[michiels]

Intercity and Dokku already support deploying via CI services. The simplest way is to add the CI's SSH key to the server, and have the CI build git push to the Dokku remote. This is currently not documented in the site or our in-app docs. Let's do that!

[beydogan]

Reference for TravisCI: https://docs.travis-ci.com/user/deployment/custom/