Fixes #17 Product Listing Resource added

Author: tareqtms

README.md

@@ -277,6 +277,7 @@ Some resources are available directly, some resources are only available through
 - Product -> Variant -> [Metafield](https://help.shopify.com/api/reference/metafield)
 - Product -> [Event](https://help.shopify.com/api/reference/event/)
 - Product -> [Metafield](https://help.shopify.com/api/reference/metafield)
+- [ProductListing](https://help.shopify.com/api/reference/sales_channels/productlisting)
 - [ProductVariant](https://help.shopify.com/api/reference/product_variant)
 - ProductVariant -> [Metafield](https://help.shopify.com/api/reference/metafield)
 - [RecurringApplicationCharge](https://help.shopify.com/api/reference/recurringapplicationcharge)
@@ -380,6 +381,10 @@ The custom methods are specific to some resources which may not be available for
     - [calculate()](https://help.shopify.com/api/reference/refund#calculate)
     Calculate a Refund.
 
+- ProductListing ->
+    - [productIds()](https://help.shopify.com/api/reference/sales_channels/productlisting#product_ids)
+    Retrieve product_ids that are published to your app.
+    
 - RecurringApplicationCharge -> 
     - [activate()](https://help.shopify.com/api/reference/recurringapplicationcharge#activate)
     Activate a recurring application charge

lib/ProductListing.php

@@ -0,0 +1,26 @@
+<?php
+/**
+ * Created by PhpStorm.
+ * @author Tareq Mahmood <tareqtms@yahoo.com>
+ * Created at: 6/2/18 1:38 PM UTC+06:00
+ *
+ * @see https://help.shopify.com/api/reference/sales_channels/productlisting Shopify API Reference for Shipping Zone
+ */
+
+namespace PHPShopify;
+
+
+class ProductListing extends ShopifyResource
+{
+    /**
+     * @inheritDoc
+     */
+    protected $resourceKey = 'product_listing';
+
+    /**
+     * @inheritDoc
+     */
+    protected $customGetActions = array (
+        'product_ids' => 'productIds',
+    );
+}