Document use of joachim-n/drupal-core-development-project for core development

[rfay]

DDEV's Drupal Core picker/template with start.coder.ddev.com uses https://github.com/joachim-n/drupal-core-development-project and it uses it just like any other Drupal development project would.

But we need to point that out in the template docs and comments at least on the templates page,

• Add info about https://github.com/joachim-n/drupal-core-development-project to template comment/docs that are displayed on the templates page
• Add to WELCOME.txt referring to this
• Show this information in the setup log
• Improve the README.md, https://coder.ddev.com/templates/drupal-core/docs

[rfay]

We're using the amateescu scaffold now, closing