Document StoreKit 1 vs StoreKit 2 usage in iOS module README (#288)

* Initial plan

* docs: document StoreKit 1 vs StoreKit 2 usage in iOS module README

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* docs: add Kotlin note clarifying no special configuration needed for gdx-pay

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---------

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Author: Copilot

gdx-pay-iosrobovm-apple/README.md

@@ -1,15 +1,71 @@
 # InApp purchasing implementation for Apple (iOS/RoboVM)
 
-### Dependencies
+This module provides two `PurchaseManager` implementations for iOS:
+
+| Implementation | API | Minimum iOS version |
+|---|---|---|
+| `PurchaseManageriOSApple` | StoreKit 1 | iOS 7+ |
+| `PurchaseManageriOSApple2` | StoreKit 2 | iOS 15+ |
+
+## Choosing between StoreKit 1 and StoreKit 2
+
+**StoreKit 2** (`PurchaseManageriOSApple2`) is the recommended implementation for new projects. It uses Apple's modern StoreKit 2 Swift-based API (via [RoboVM StoreKit 2 bindings](https://github.com/MobiVM/robovm-cocoatouch-swift)) and provides:
+
+* Improved subscription handling, including eligibility checks for introductory offers via `isEligibleForIntroOffer()`
+* A modern async-based API under the hood
+
+**StoreKit 1** (`PurchaseManageriOSApple`) should be used if your app needs to support iOS versions below 15.
+
+If your app targets a range of iOS versions, you can select the implementation at runtime based on the device's iOS version (see [Instantiation](#instantiation) below).
+
+## Dependencies
 
      implementation "com.badlogicgames.gdxpay:gdx-pay-iosrobovm-apple:$gdxPayVersion"
 
-### Instantiation
+This single dependency includes both `PurchaseManageriOSApple` (StoreKit 1) and `PurchaseManageriOSApple2` (StoreKit 2).
+
+The StoreKit 2 implementation depends on the RoboVM StoreKit 2 bindings, which are included as a transitive dependency:
+
+     com.mobidevelop.robovm:robopods-swift-storekit2
+
+If your app only uses StoreKit 1 and you want to exclude the StoreKit 2 transitive dependency, you can do so in your Gradle configuration:
+
+     implementation("com.badlogicgames.gdxpay:gdx-pay-iosrobovm-apple:$gdxPayVersion") {
+         exclude group: 'com.mobidevelop.robovm', module: 'robopods-swift-storekit2'
+     }
+
+### Note for Kotlin users
+
+Both `PurchaseManageriOSApple` and `PurchaseManageriOSApple2` work with Java and Kotlin without any special configuration. All async operations and callbacks are handled internally by the `PurchaseManager` implementation — you interact with it through the standard `PurchaseObserver` interface regardless of your language.
+
+Separately, there is also a Kotlin coroutine wrapper available (`com.mobidevelop.robovm:robopods-swift-storekit2-kt`) for projects that want to use the StoreKit 2 API directly without gdx-pay. This is **not** needed when using gdx-pay.
+
+## Instantiation
+
+### Using StoreKit 1 only
 
 Add this to your `IOSLauncher`:
 
     game.purchaseManager = new PurchaseManageriOSApple();
 
+### Using StoreKit 2 only
+
+Add this to your `IOSLauncher`:
+
+    game.purchaseManager = new PurchaseManageriOSApple2();
+
+### Selecting at runtime based on iOS version
+
+If your app supports both older and newer iOS versions, you can choose the implementation at runtime:
+
+    import org.robovm.apple.foundation.Foundation;
+
+    if (Foundation.getMajorSystemVersion() >= 15) {
+        game.purchaseManager = new PurchaseManageriOSApple2();
+    } else {
+        game.purchaseManager = new PurchaseManageriOSApple();
+    }
+
 ## Testing
 Next to other ways, I find the easiest way to test the IAP the following: