Add Subscription Group section, documenting usage of verifySubscriptions

README.md

@@ -477,6 +477,36 @@ SwiftyStoreKit.purchaseProduct(productId, atomically: true) { result in
 }
 ```
 
+### Subscription Groups
+
+From [Apple Docs - Offering Subscriptions](https://developer.apple.com/app-store/subscriptions/):
+
+> A subscription group is a set of in-app purchases that you can create to provide users with a range of content offerings, service levels, or durations to best meet their needs. Users can only buy one subscription within a subscription group at a time. If users would want to buy more that one type of subscription — for example, to subscribe to more than one channel in a streaming app — you can put these in-app purchases in different subscription groups.
+
+You can verify all subscriptions within the same group with the `verifySubscriptions` method:
+
+```swift
+let appleValidator = AppleReceiptValidator(service: .production, sharedSecret: "your-shared-secret")
+SwiftyStoreKit.verifyReceipt(using: appleValidator) { result in
+    switch result {
+    case .success(let receipt):
+        let productIds = Set([ "com.musevisions.SwiftyStoreKit.Weekly",
+                               "com.musevisions.SwiftyStoreKit.Monthly",
+                               "com.musevisions.SwiftyStoreKit.Yearly" ])
+        let purchaseResult = SwiftyStoreKit.verifySubscriptions(productIds: productIds, inReceipt: receipt)
+        switch purchaseResult {
+        case .purchased(let expiryDate, let items):
+            print("\(productIds) are valid until \(expiryDate)\n\(items)\n")
+        case .expired(let expiryDate, let items):
+            print("\(productIds) are expired since \(expiryDate)\n\(items)\n")
+        case .notPurchased:
+            print("The user has never purchased \(productIds)")
+        }
+    case .error(let error):
+        print("Receipt verification failed: \(error)")
+    }
+}
+```
 
 ## Notes
 The framework provides a simple block based API with robust error handling on top of the existing StoreKit framework. It does **NOT** persist in app purchases data locally. It is up to clients to do this with a storage solution of choice (i.e. NSUserDefaults, CoreData, Keychain).