Write README.md and module description

@grevil would you please do this by time? No urgency ;)

Once this is merged, I can copy the README to the module page (changing it up a bit of course to better fit the webpage setting).

Hm, the README doesn't go deep enough for me. We should have submodule specific sections, with their respective requirements, and what they do and how to do basic configuration.

But I love the idea of a single README! Good stuff :)

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;
}

We should have submodule specific sections, with their respective requirements, and what they do and how to do basic configuration.

In that case, we should opt to have a README for each of the submodules within their respective module folders, instead of cramming everything into here, right? At least, a lot of complex modules like Webform do it that way.
In a sense, I have already prepared us for this approach by keeping the main module description general and listing submodule features rather than explaining them in detail.

@grevil, what do you think about this? I will add the help hook in the meantime.

Re #12 I agree. I also think modularity and a specific README.md per submodule might be the better choice, instead of a huge one in the main module. The submodules should better describe themselves.

Anyway we should not waste too much time on this. @Grevil maybe AI can generate them? Would that speed things up?