Add documentation about hook_scald_atom_quickadd()

Implementation example:

/**
 * Implementation of hook_scald_atom_quickadd().
 */
function scald_image_scald_atom_quickadd() {
  $type = array(
    'type' => 'Image',
    'addpath' => 'node/add/story',
    'class' => 'rf-frame-child rf-frame-size[650,650]',
  );  
  return array($type);
}