Add custom fields to Order email of Woocommerce

order-emails.md

Add the field to order emails:

 add_filter('woocommerce_email_order_meta_keys', 'my_woocommerce_email_order_meta_keys');

 function my_woocommerce_email_order_meta_keys( $keys ) {
$keys['How did you hear about us?'] = 'hear_about_us';
return $keys;
 }