drupal/core is implicitly allowed by scaffold

Problem/Motivation

The README for the Composer Scaffold plugin uses drupal/core as an example allowed-package. This is a poor example, because drupal/core is implicitly allowed and explicitly listing it has no effect. Using drupal/core as an example allowed-package misleads people into thinking that it has to be explicitly listed.

Proposed resolution

Either clearly state in the examples that drupal/core is implicitly allowed, or choose a different example package