Add README

Author: microbit-sam

README.md

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+# micro:bit Partial Flashing Library for Android
+
+This library provides partial flashing capabilities to an Android application. It will process a hex file created in MakeCode and flash only the MakeCode script.
+
+To modularize the code, information is passed to and from the library using Intents.
+
+## Including the lib in an application
+
+Create a class that extends the base service:
+
+```
+package com.microbitreactnative.pf;
+
+import android.app.Activity;
+
+import org.microbit.android.partialflashing.PartialFlashingBaseService;
+
+import com.microbitreactnative.NotificationActivity;
+
+public class PartialFlashingService extends PartialFlashingBaseService {
+
+    @Override
+    protected Class<? extends Activity> getNotificationTarget() {
+        return NotificationActivity.class;
+    }
+}
+``` 
+
+## To start a partial flash
+
+Send an Intent with the micro:bit's device address (scanning and bonding is not handled by this library) and the file path of the hex file to flash.
+
+```
+    MainApplication application = MainApplication.getApp();
+
+    Log.v("MicrobitDFU", "Start Partial Flash");
+
+    // final Intent service = new Intent(application, PartialFlashingService.class);
+    final Intent service = new Intent(application, PartialFlashingService.class);
+    service.putExtra("deviceAddress", deviceAddress);
+    service.putExtra("filepath", filePath); // a path or URI must be provided.
+
+    application.startService(service);
+```
+
+## Receiving progress updates
+
+Currently the library only sends progress as percentage updates (0-100%). These are broadcast during the flashing process and can be obtained using a LocalBroadcastManager.
+
+An example that forwards the information to a React Native app:
+
+```
+...
+
+  public static final String BROADCAST_PROGRESS_PF = "org.microbit.android.partialflashing.broadcast.BROADCAST_PROGRESS";
+  public static final String EXTRA_PROGRESS_PF = "org.microbit.android.partialflashing.extra.EXTRA_PROGRESS";
+
+  private ReactContext mReactContext;
+  private LocalBroadcastReceiver  mLocalBroadcastReceiver;
+
+  public MicrobitDFUModule(ReactApplicationContext reactContext) {
+    super(reactContext);
+    this.mReactContext = reactContext;
+    this.mLocalBroadcastReceiver = new LocalBroadcastReceiver();
+    LocalBroadcastManager localBroadcastManager = LocalBroadcastManager.getInstance(reactContext);
+    localBroadcastManager.registerReceiver(mLocalBroadcastReceiver, new IntentFilter(BROADCAST_PROGRESS_PF));
+  }
+
+  public class LocalBroadcastReceiver extends BroadcastReceiver {
+         @Override
+         public void onReceive(Context context, Intent intent) {
+               // Doesn't need to be precise so using int
+               int percentage = intent.getIntExtra(EXTRA_PROGRESS_PF, 0);
+
+               mReactContext.getJSModule(DeviceEventManagerModule.RCTDeviceEventEmitter.class)
+                      .emit("flashingProgress", percentage);
+         }
+  }
+
+...
+```