Added link to Technical Note 2413 (In-App Purchase FAQ Troubleshooting)

README.md

@@ -344,6 +344,7 @@ Note that the pre-registered in app purchases in the demo apps are for illustrat
 * [Apple - Validating Receipts Locally](https://developer.apple.com/library/content/releasenotes/General/ValidateAppStoreReceipt/Chapters/ValidateLocally.html)
 * [Apple - Offering Subscriptions](https://developer.apple.com/app-store/subscriptions/)
 * [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
 
 I have also written about building SwiftyStoreKit on Medium: