feat(sdk): add documentation for order notes

src/documentation/50.php-sdk.md

@@ -643,6 +643,41 @@ $orderCollection->push($order);
 $savedOrderCollection = $orderCollection->save();
 ```
 
+### Adding order notes to an order
+
+It is possible to add notes to an order. These notes are visible in the backoffice.
+A note can be either a customer note or a webshop note. A customer note is
+always created in the checkout process and a webshop note can be created in the
+admin panel of an e-commerce platform.
+
+To add a note to an order, simply create
+an `OrderNote` and add it to an `OrderNoteCollection`. Then, save the collection
+to our API.
+
+```php
+use MyParcelNL\Sdk\src\Model\Fulfilment\OrderNote;
+use MyParcelNL\Sdk\src\Collection\Fulfilment\OrderNoteCollection;
+
+$orderNoteCollection = new OrderNoteCollection();
+$orderNote           = new OrderNote([
+    'orderUuid' => 'uuid_of_order', // You need the uuid generated by our API.
+    'note'      => 'This is a note',
+    'author'    => 'webshop', // This can be either 'webshop' or 'customer'.
+])
+
+$orderNoteCollection->push($orderNote);
+
+// This will send the order note to our API. Thanks to the uuid, the note will
+// be placed in the correct order.
+$savedOrderNoteCollection = $orderNoteCollection->save();
+```
+
+::: note
+Note: The uuid of an order can be found in the response of the `save` method of
+the `OrderCollection`. Therefore, you need to save the order first before you
+can add notes to it.
+:::
+
 ### Setting up webhooks
 
 You can use our webhooks to keep track of changes in shipments, labels and your