Add a custom message on install of Drupal 9 module (module development for better sitebuilder experience)

See https://www.drupal.org/docs/develop/managing-a-drupalorg-theme-module-or-distribution-project/documenting-your-project/module-documentation-guidelines#s-recommended-practices

And https://www.drupal.org/project/drupal/issues/134849#comment-14671020