Added Apple recommendations on how to restore previous purchases.

README.md

@@ -128,6 +128,16 @@ SwiftyStoreKit.purchaseProduct("com.musevisions.SwiftyStoreKit.Purchase1", quant
 
 ### Restore previous purchases
 
+According to [Apple - Restoring Purchased Products](https://developer.apple.com/library/content/documentation/NetworkingInternet/Conceptual/StoreKitGuide/Chapters/Restoring.html#//apple_ref/doc/uid/TP40008267-CH8-SW9):
+
+> In most cases, all your app needs to do is refresh its receipt and deliver the products in its receipt. The refreshed receipt contains a record of the user’s purchases in this app, on this device or any other device.
+
+> Restoring completed transactions creates a new transaction for every completed transaction the user made, essentially replaying history for your transaction queue observer.
+
+See the **Receipt Verification** section below for how to restore previous purchases using the receipt.
+
+This section shows how to restore completed transactions with the `restorePurchases` method instead.
+
 * **Atomic**: to be used when the content is delivered immediately.
 
 ```swift
@@ -442,6 +452,7 @@ 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 - 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)
 * [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