Add a README.md and add an example for bulk-updating user preference

Problem/Motivation

Please add a README.md with general information about the module (may use the text from the module page in large parts) and add a code-example for bulk-updating the user setting to a certain route:

AI says this might work, but please test first, if it really does:

  $new_route = 'TODO';

  $user_storage = \Drupal::entityTypeManager()->getStorage('user');
  // Get all users
  $user_ids = $user_storage->getQuery()
    ->execute();

  /** @var \Drupal\user\UserDataInterface $user_data */
  $user_data = \Drupal::service('user.data');

  foreach ($user_ids as $uid) {
    $current = $user_data->get('user_preference_login_redirect', $uid, 'login_redirect_route');
    if (empty($current)) {
      $user_data->set('user_preference_login_redirect', $uid, 'login_redirect_route', $new_route);
    }
  }

  return t('Default login redirect route "front.page" set for all users without a preference.');

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes