docs: Add warning about iOS support to docs for document scanner (#227)

Author: trevor-coleman

docs/document-scanner/index.md

@@ -10,6 +10,19 @@ title: Getting Started
 This is an expo module that lets you use
 the [MLKit Document Scanner](https://developers.google.com/ml-kit/vision/doc-scanner) library in your Expo app.
 
+:::warning
+ML Kit does not provide a Document Scanning module for iOS, so this feature is **not supported on iOS**.
+
+This limitation comes from Google’s ML Kit itself and cannot be addressed by this library.
+
+If you need document scanning on iOS, here are some popular React Native packages that support it:
+
+- [@dariyd/react-native-document-scanner](https://www.npmjs.com/package/%40dariyd/react-native-document-scanner) — iOS (VisionKit) + Android (ML Kit)
+- [react-native-document-scanner-plugin](https://www.npmjs.com/package/react-native-document-scanner-plugin) — lightweight, supports both platforms
+- [@thegrizzlylabs/react-native-genius-scan](https://www.npmjs.com/package/@thegrizzlylabs/react-native-genius-scan) — commercial, full-featured scanning SDK
+:::
+
+
 ## Installation
 
 Install like any other npm package: