added usage to the GiftCardAdjustment class

Author: whobutsb

lib/GiftCardAdjustment.php

@@ -6,6 +6,26 @@
  *
  * @see https://shopify.dev/docs/api/admin-rest/2023-01/resources/gift-card-adjustment Shopify API Reference for Gift Card Adjustment
  * @note - requires gift_card_adjustments access scope enabled by Shopify Support
+ *
+ * @usage: 
+ *
+    $shopify = \PHPShopify\ShopifySDK::config($config);
+
+    $gift_card_id = 88888888888;
+
+    $gift_card_adjustment_id = 999999999999;
+
+    // Get all gift card adjustments
+    $shopify->GiftCard($gift_card_id)->Adjustment()->get();
+
+    // Get a single gift card adjustment
+    $shopify->GiftCard($gift_card_id)->Adjustment($gift_card_adjustment_id)->get();
+
+    // Create a gift card adjustment
+    $shopify->GiftCard($gift_card_id)->Adjustment()->post([
+        'amount' => 5,
+        'note' => 'Add $5 to gift card'
+    ]);
  */
 
 namespace PHPShopify;