Add information which versions to use (7.x, 2.x) and path details

Also it's not totally clear which library path structure is expected.

Drupal 7 expects the following:

if (file_exists($dir . '/iframe-resizer/js/iframeResizer.contentWindow.js') && file_exists($dir . '/iframe-resizer/js/iframeResizer.js')) {
      return $dir . '/iframe-resizer';
    }

Seems currently version 4.x is expected?