README update

Author: uerceg

README.md

@@ -60,6 +60,7 @@ Read this in other languages: [English][en-readme], [中文][zh-readme], [日本
    * [Push token (uninstall tracking)](#ad-push-token)
    * [Attribution callback](#ad-attribution-callback)
    * [Ad revenue tracking](#ad-ad-revenue)
+   * [Subscription tracking](#ad-subscriptions)
    * [Session and event callbacks](#ad-session-event-callbacks)
    * [User attribution](#ad-user-attribution)
    * [Device IDs](#ad-device-ids)
@@ -592,6 +593,109 @@ Currently we support the following `source` parameter values:
 
 - `AdjustConfig.AdjustAdRevenueSourceMopub` - represents the [MoPub mediation platform][sdk2sdk-mopub]
 
+### <a id="af-subscriptions"></a>Subscription tracking
+
+**Note**: This feature is only available in the SDK v4.22.0 and above.
+
+You can track App Store and Play Store subscriptions and verify their validity with the Adjust SDK. After a subscription has been successfully purchased, make the following call to the Adjust SDK:
+
+**For App Store subscription:**
+
+```csharp
+AdjustAppStoreSubscription subscription = new AdjustAppStoreSubscription(
+    price,
+    currency,
+    transactionId,
+    receipt);
+subscription.setTransactionDate(transactionDate);
+subscription.setSalesRegion(salesRegion);
+
+Adjust.trackAppStoreSubscription(subscription);
+```
+
+**For Play Store subscription:**
+
+```csharp
+AdjustPlayStoreSubscription subscription = new AdjustPlayStoreSubscription(
+    price,
+    currency,
+    sku,
+    orderId,
+    signature,
+    purchaseToken);
+subscription.setPurchaseTime(purchaseTime);
+
+Adjust.trackPlayStoreSubscription(subscription);
+```
+
+Subscription tracking parameters for App Store subscription:
+
+- [price](https://developer.apple.com/documentation/storekit/skproduct/1506094-price?language=objc)
+- currency (you need to pass [currencyCode](https://developer.apple.com/documentation/foundation/nslocale/1642836-currencycode?language=objc) of the [priceLocale](https://developer.apple.com/documentation/storekit/skproduct/1506145-pricelocale?language=objc) object)
+- [transactionId](https://developer.apple.com/documentation/storekit/skpaymenttransaction/1411288-transactionidentifier?language=objc)
+- [receipt](https://developer.apple.com/documentation/foundation/nsbundle/1407276-appstorereceipturl)
+- [transactionDate](https://developer.apple.com/documentation/storekit/skpaymenttransaction/1411273-transactiondate?language=objc)
+- salesRegion (you need to pass [countryCode](https://developer.apple.com/documentation/foundation/nslocale/1643060-countrycode?language=objc) of the [priceLocale](https://developer.apple.com/documentation/storekit/skproduct/1506145-pricelocale?language=objc) object)
+
+Subscription tracking parameters for Play Store subscription:
+
+- [price](https://developer.android.com/reference/com/android/billingclient/api/SkuDetails#getpriceamountmicros)
+- [currency](https://developer.android.com/reference/com/android/billingclient/api/SkuDetails#getpricecurrencycode)
+- [sku](https://developer.android.com/reference/com/android/billingclient/api/Purchase#getsku)
+- [orderId](https://developer.android.com/reference/com/android/billingclient/api/Purchase#getorderid)
+- [signature](https://developer.android.com/reference/com/android/billingclient/api/Purchase#getsignature)
+- [purchaseToken](https://developer.android.com/reference/com/android/billingclient/api/Purchase#getpurchasetoken)
+- [purchaseTime](https://developer.android.com/reference/com/android/billingclient/api/Purchase#getpurchasetime)
+
+**Note:** Subscription tracking API offered by Adjust SDK expects all parameters to be passed as `string` values. Parameters described above are the ones which API exects you to pass to subscription object prior to tracking subscription. There are various libraries which are handling in app purchases in Unity and each one of them should return information described above in some form upon successfully completed subscription purchase. You should locate where these parameters are placed in response you are getting from library you are using for in app purchases, extract those values and pass them to Adjust API as string values.
+
+Just like with event tracking, you can attach callback and partner parameters to the subscription object as well:
+
+**For App Store subscription:**
+
+```csharp
+AdjustAppStoreSubscription subscription = new AdjustAppStoreSubscription(
+    price,
+    currency,
+    transactionId,
+    receipt);
+subscription.setTransactionDate(transactionDate);
+subscription.setSalesRegion(salesRegion);
+
+// add callback parameters
+subscription.addCallbackParameter("key", "value");
+subscription.addCallbackParameter("foo", "bar");
+
+// add partner parameters
+subscription.addPartnerParameter("key", "value");
+subscription.addPartnerParameter("foo", "bar");
+
+Adjust.trackAppStoreSubscription(subscription);
+```
+
+**For Play Store subscription:**
+
+```csharp
+AdjustPlayStoreSubscription subscription = new AdjustPlayStoreSubscription(
+    price,
+    currency,
+    sku,
+    orderId,
+    signature,
+    purchaseToken);
+subscription.setPurchaseTime(purchaseTime);
+
+// add callback parameters
+subscription.addCallbackParameter("key", "value");
+subscription.addCallbackParameter("foo", "bar");
+
+// add partner parameters
+subscription.addPartnerParameter("key", "value");
+subscription.addPartnerParameter("foo", "bar");
+
+Adjust.trackPlayStoreSubscription(subscription);
+```
+
 ### <a id="ad-session-event-callbacks"></a>Session and event callbacks
 
 You can set up callbacks to notify you of successful and failed events and/or sessions.