Provide the default form for setting options.
Overrides PluginBase::buildOptionsForm
public function buildOptionsForm(&$form, &$form_state) {
parent::buildOptionsForm($form, $form_state);
$form['offset'] = array(
'#type' => 'textfield',
'#title' => t('Offset (number of items to skip)'),
'#description' => t('For example, set this to 3 and the first 3 items will not be displayed.'),
'#default_value' => $this->options['offset'],
);
}