Add note to quickstart about deferring database initialization
The quick-start is supposed to be just that -- a quick start. It is nowhere near comprehensive.

The [database](http://docs.peewee-orm.com/en/latest/peewee/database.html) documentation contains tons of information:

* [run-time database configuration](http://docs.peewee-orm.com/en/latest/peewee/database.html#run-time-database-configuration)
* [dynamically defining a database](http://docs.peewee-orm.com/en/latest/peewee/database.html#dynamically-defining-a-database)
* [setting the database at run-time](http://docs.peewee-orm.com/en/latest/peewee/database.html#setting-the-database-at-run-time)
* [testing peewee applications](http://docs.peewee-orm.com/en/latest/peewee/database.html#testing)
I'm new to peewee, and must admit I spent way too much time trying to figure out how to make my application use a temporary database during tests.  I'm generating the database path with [appdirs](https://github.com/ActiveState/appdirs) and changing the environment during tests with [pytest's monkeypatch](https://docs.pytest.org/en/latest/reference.html#_pytest.monkeypatch.MonkeyPatch.setenv).  Run-time database configuration was exactly what I needed, I just didn't know to search for that term.  Eventually I found the documentation for that, but I think it's important enough to make note of it in the quickstart.