Add link to Apple Docs

README.md

@@ -512,6 +512,7 @@ Note that the pre-registered in app purchases in the demo apps are for illustrat
 * [Apple - Working with Subscriptions](https://developer.apple.com/library/content/documentation/NetworkingInternet/Conceptual/StoreKitGuide/Chapters/Subscriptions.html#//apple_ref/doc/uid/TP40008267-CH7-SW6)
 * [Apple - Offering Subscriptions](https://developer.apple.com/app-store/subscriptions/)
 * [Apple - Restoring Purchased Products](https://developer.apple.com/library/content/documentation/NetworkingInternet/Conceptual/StoreKitGuide/Chapters/Restoring.html#//apple_ref/doc/uid/TP40008267-CH8-SW9)
+* [Apple - Testing In-App Purchase Products](https://developer.apple.com/library/content/documentation/LanguagesUtilities/Conceptual/iTunesConnectInAppPurchase_Guide/Chapters/TestingInAppPurchases.html): includes info on duration of subscriptions in sandbox mode
 * [objc.io - Receipt Validation](https://www.objc.io/issues/17-security/receipt-validation/)
 * [Apple TN 2413 - Why are my product identifiers being returned in the invalidProductIdentifiers array?](https://developer.apple.com/library/content/technotes/tn2413/_index.html#//apple_ref/doc/uid/DTS40016228-CH1-TROUBLESHOOTING-WHY_ARE_MY_PRODUCT_IDENTIFIERS_BEING_RETURNED_IN_THE_INVALIDPRODUCTIDENTIFIERS_ARRAY_)
 * [Invalid Product IDs](http://troybrant.net/blog/2010/01/invalid-product-ids/): Checklist of common mistakes