title: Adding a receipt page on woocommerce
body: <p>I am developing a payment gateway for woocommerce.
  I read many tutorial of how I can manage to do it but I can't figure out what this line of code mean:</p>
  
  <pre><code>add_action( 'woocommerce_receipt_paypal', array( $this, 'receipt_page' ) );
  </code></pre>
  
  <p>Especially, there is no function called woocommerce_receipt_paypal and how can I reproduce it in my plugin.</p>