Update README.md

Author: microbit-sam

README.md

@@ -55,6 +55,7 @@ An example that forwards the information to a React Native app:
   public static final String BROADCAST_START = "org.microbit.android.partialflashing.broadcast.BROADCAST_START";
   public static final String BROADCAST_COMPLETE = "org.microbit.android.partialflashing.broadcast.BROADCAST_COMPLETE";
   public static final String EXTRA_PROGRESS = "org.microbit.android.partialflashing.extra.EXTRA_PROGRESS";
+  public static final String BROADCAST_PF_FAILED = "org.microbit.android.partialflashing.broadcast.BROADCAST_PF_FAILED";
 
   private ReactContext mReactContext;
   private LocalBroadcastReceiver  mLocalBroadcastReceiver;