Adds a notice for multiple twig renders

[NielsdeBlaauw]

Using clarkson_template::render_twig in a foreach
is a lot more expensive than including a partial in
a twig template, which is the most common use-case.

This user notice allows for easier tracking of this
performance bottleneck.

The test exsits of this code, common in some current projects:

    public static function getItems(){
      $clarkson_templates = \Clarkson_Core_Templates::get_instance();
      $path = get_template_directory() . '/templates/title.twig';

      $html = '';

      $items = self::get_many(array('posts_per_page'=>100));
      foreach($items as $item){
        $data = array("post"=>$item);
        $html .= $clarkson_templates->render_twig( $path, $data, true);
      }

      return $html;
    }

{{ getItems() }}

Versus

    public static function getItems(){
      return self::get_many(array('posts_per_page'=>100));
    }

{% for item in getItems() %}
  {% include 'title.twig' with {'post':item} %}
{% endfor %}

Multiple render_twig calls
Requests per second: 1.44 #/sec

Single render_twig call with includes
Requests per second: 3.22 #/sec

mutiple_render.ab.txt
single_render.ab.txt