README doesn't need to mention `db:test:prepare`

[Mark D. Blackwell]

The README recommends users set up their test environment with db:test:prepare.

However, automatically now during testing, Rails itself does db:test:prepare, as revealed by bundle exec rake --trace. So, this step in the docs doesn't add anything.

This is similar to an issue in another Rails Example App.

[Mark D. Blackwell]

Users need to do this before invoking Cucumber or RSpec without Rake, so closing.