Add In-App Purchase FAQ to README

README.md

@@ -470,6 +470,7 @@ Note that the pre-registered in app purchases in the demo apps are for illustrat
 ## Essential Reading
 * [Apple - WWDC16, Session 702: Using Store Kit for In-app Purchases with Swift 3](https://developer.apple.com/videos/play/wwdc2016/702/)
 * [Apple - TN2387: In-App Purchase Best Practices](https://developer.apple.com/library/content/technotes/tn2387/_index.html)
+* [Apple - TN2413: In-App Purchase FAQ](https://developer.apple.com/library/content/technotes/tn2413/_index.html)
 * [Apple - About Receipt Validation](https://developer.apple.com/library/content/releasenotes/General/ValidateAppStoreReceipt/Introduction.html)
 * [Apple - Receipt Validation Programming Guide](https://developer.apple.com/library/content/releasenotes/General/ValidateAppStoreReceipt/Chapters/ReceiptFields.html#//apple_ref/doc/uid/TP40010573-CH106-SW1)
 * [Apple - Validating Receipts Locally](https://developer.apple.com/library/content/releasenotes/General/ValidateAppStoreReceipt/Chapters/ValidateLocally.html)