Add receive callback mode using DIO0

examples/LoRaReceiverCallback/LoRaReceiverCallback.ino

@@ -0,0 +1,39 @@
+#include <SPI.h>
+#include <LoRa.h>
+
+void setup() {
+  Serial.begin(9600);
+  while (!Serial);
+
+  Serial.println("LoRa Receiver Callback");
+
+  if (!LoRa.begin(915E6)) {
+    Serial.println("Starting LoRa failed!");
+    while (1);
+  }
+
+  // register the receive callback
+  LoRa.onReceive(onReceive);
+
+  // put the radio into receive mode
+  LoRa.receive();
+}
+
+void loop() {
+  // do nothing
+}
+
+void onReceive(int packetSize) {
+  // received a packet
+  Serial.print("Received packet '");
+
+  // read packet
+  for (int i = 0; i < packetSize; i++) {
+    Serial.print((char)LoRa.read());
+  }
+
+  // print RSSI of packet
+  Serial.print("' with RSSI ");
+  Serial.println(LoRa.packetRssi());
+}
+