Add inclusion of a settings.local.php file in settings.php

The Drupal.org infrastructure uses this by default and so do lots of Drupal shops, so why can't we add this snippet to settings.php (and settings.default.php) by default?

/**
 * Include a local settings file if it exists.
 */
$local_settings = dirname(__FILE__) . '/settings.local.php';
if (file_exists($local_settings)) {
  include $local_settings;
}

This could also be a tiny step forwards for local configuration management.