add docs for tag event stream support

Author: dangfan

README.md

@@ -38,6 +38,64 @@ To use this plugin on Android, you also need to:
 
 * Add [android.permission.NFC](https://developer.android.com/reference/android/Manifest.permission.html#NFC) to your `AndroidManifest.xml`.
 
+To receive NFC tag events even when your app is in the foreground, you can set up tag event stream support:
+
+1. Create a custom Activity that extends `FlutterActivity` in your Android project:
+
+```kotlin
+package your.package.name
+
+import android.app.PendingIntent
+import android.content.Intent
+import android.nfc.NfcAdapter
+import android.nfc.Tag
+import io.flutter.embedding.android.FlutterActivity
+import im.nfc.flutter_nfc_kit.FlutterNfcKitPlugin
+
+class MainActivity : FlutterActivity() {
+    override fun onResume() {
+        super.onResume()
+        val adapter: NfcAdapter? = NfcAdapter.getDefaultAdapter(this)
+        val pendingIntent: PendingIntent = PendingIntent.getActivity(
+            this, 0, Intent(this, javaClass).addFlags(Intent.FLAG_ACTIVITY_SINGLE_TOP), PendingIntent.FLAG_MUTABLE
+        )
+        // See https://developer.android.com/reference/android/nfc/NfcAdapter#enableForegroundDispatch(android.app.Activity,%20android.app.PendingIntent,%20android.content.IntentFilter[],%20java.lang.String[][]) for details 
+        adapter?.enableForegroundDispatch(this, pendingIntent, null, null)
+    }
+
+    override fun onPause() {
+        super.onPause()
+        val adapter: NfcAdapter? = NfcAdapter.getDefaultAdapter(this)
+        adapter?.disableForegroundDispatch(this)
+    }
+
+    override fun onNewIntent(intent: Intent) {
+        val tag: Tag? = intent.getParcelableExtra(NfcAdapter.EXTRA_TAG)
+        if (tag != null) {
+            FlutterNfcKitPlugin.handleTag(tag)
+        }
+    }
+}
+```
+
+2. Update your `AndroidManifest.xml` to use this activity instead of the default Flutter activity.
+
+3. In your Flutter code, listen to the tag event stream:
+
+```dart
+@override
+void initState() {
+  super.initState();
+  // Listen to NFC tag events
+  FlutterNfcKit.tagStream.listen((tag) {
+    print('Tag detected: ${tag.id}');
+    // Process the tag
+  });
+}
+```
+
+This will allow your app to receive NFC tag events through a stream, which is useful for scenarios where you need continuous tag reading or want to handle tags even when your app is in the foreground but not actively polling.
+
 ### iOS
 
 This plugin now supports Swift package manager, and requires iOS 13+.