Add user docs

[malankriel]

mkdocs setup for instructions on how to set up, etc etc

[rudolfbyker]

@heetbeet as discussed today, it's best to have everything specified in a single configuration file, like application.yaml (YAML is a great file format for human-written config, so I would not change that), and then just document the config file properly. The config file is then front-and-centre both in the documentation and in the user experience. This is how most build tools work, and how most tools in the JS/Node world work, how mypy works, how mkdocs itself works. Having it scan for scripts having specific names is not that great. Rather, let the user specify the pre and post scripts in application.yaml.