NEW: More helpful message for 'sake dev/build' on new envs.

This change alters the no-db message on cli execution to give a bit more of a helpful set-up instruction.

The main motivation for this is so that composer can be set to run dev/build on post-install and post-update.  With that feature added, this will ensure that users installing with composer create-project won't be left in the dark.

That change is suggested in https://github.com/silverstripe/silverstripe-installer/pull/42

An improvement on this would be a shell script that interactively asked for details to populate this file with, but one step at a time.