Doc updates

[ckoerber]

A list of issues and possible improvements to be included in next versions of the documentation.

• [x] Better example and links how to set up models in the step-by-step introduction
• [x] Change file extension of db from .yaml to .sqlite in the intro db-config.yaml
• [x] Write a section about possible development issues (e.g., populate() not reentrant when imports fail).

[ckoerber]

The docs have been updated with cb3404f

The populate message is delegated to the groups: https://groups.google.com/forum/#!topic/espressodb/3A0OmnIjTxA

[ckoerber]

Closed with #48