Yii: How to add a simple checkbox in a view

I want to do add a checkbox to a view using the Yii2 framework. Using HTML, JavaScript or Angular is very easy but I don't understand how to do it with Yii2.

I have a username input in the view called _form.php:

<div class="col-md-6">
    <?= $form->field($model, 'username')->textInput(['maxlength' => true]) ?>
</div>

And now I need a checkbox.

Is there any Yii documentation showing all its components?