Write module description and README.md

The proposed README is still missing a lot of points and not following the Drupal Standard very well.

It also seems to be a more or less direct Markdown translation of the module page, which is not what we wanted since we wanted to redo the module page too.

I will redo it quickly.

Once this is merged, I can use the README as a template for the module page and improve it a bit (including our support snippets at the end of the page).

Commented the MR.

And don't forget the README help page hook:

/**
 * Implements hook_help().
 */
function my_module_help($route_name, RouteMatchInterface $route_match) {
  switch ($route_name) {
    case 'help.page.my_module':
      $text = file_get_contents(__DIR__ . '/README.md');
      if (!\Drupal::moduleHandler()->moduleExists('markdown')) {
        return '<pre>' . Html::escape($text) . '</pre>';
      }
      else {
        // Use the Markdown filter to render the README.
        $filter_manager = \Drupal::service('plugin.manager.filter');
        $settings = \Drupal::configFactory()->get('markdown.settings')->getRawData();
        $config = ['settings' => $settings];
        $filter = $filter_manager->createInstance('markdown', $config);
        return $filter->process($text, 'en');
      }
  }
  return NULL;
}

I have added the help hook now and also rephrased the description and configuration sections to point out that the prefix label is generated, and the separator is configurable.

@grevil, what do you think — is the new phrasing appropriate?