Add database creation to docs

[markbates]

Issue moved from gobuffalo/buffalo#833

This project is great and I can't wait to dive in further. Thanks for creating/maintaining it!

Getting started with buffalo has been relatively easy on one of my workstations but getting the database up was easily the part that had me opening a slew of tabs to google around. Where's a known good docker container that can run postgres/sqlite3/mysql? How to run the container such that it runs on the correct port, or is accessible by name, or creates a database file we can use straight away? It didn't take super long to resolve but it wasn't trivial for someone even having experience with these technologies.

[jmvbxx]

This was discussed in #buffalo today and a plan of action was suggested by @stanislas-m .

Can I please have this ticket assigned to me? It's easier for me to track my tasks that way.

Thank you!