README.txt converted to Markdown

The core/README.txt file that previously shipped with Drupal core has been converted to Markdown and renamed to core/README.md. Its focus has changed more towards shepherding new contributors into the Drupal ecosystem. The usage information it previously contained has been extracted into a separate core/USAGE.txt file.

If you are using the drupal/core-composer-scaffold plugin to exclude README.txt, you will want to update your scaffold configuration in composer.json to exclude the new file, similar to:

{
  "extra": {
    "drupal-scaffold": {
      "file-mapping": {
        "[web-root]/README.md": false
      }
    }
  }
}