Add documentation on how to use in various systems

For example, combining with Heroku and https://devcenter.heroku.com/articles/preboot. This is more important once I merge database plugin support with locking and data/schema migration separation.

Basically, you can use `preboot`-like behaviour to achieve zero downtime through:
- Run schema migrations during reboot of an application (no `preboot`)
- When there's no schema migrations, you should enable `preboot`
- Run data migrations as a separated script (maybe two new fields in migrations scripts like `dataUp` and `dataDown`, `poststart` and `postdown`, etc - open to suggestions)