Add README.md with example

README.md

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+# espasyncudplistener
+ESP32 async udp listener
+
+## Example usage
+```C++
+#include <esp_log.h>
+#include <espwifiutils.h>
+
+#include <asyncudplistener.h>
+
+namespace {
+constexpr const char * const TAG = "MYAPP";
+} // namespace
+
+AsyncUdpListener udpListener;
+
+void setup()
+{
+    constexpr const uint16_t listeningPort = 1234;
+    if (!udpListener.listen(listeningPort))
+        ESP_LOGE(TAG, "could not start listening on udp (port=%i)", listeningPort);
+}
+
+void handleUdpPacket(const UdpPacketWrapper &packet)
+{
+    ESP_LOGI(TAG, "udp response from %s : \"%.*s\"",
+             wifi_stack::toString(packet.remoteAddr()).c_str(),
+             packet.data().size(), packet.data().data());
+    // TODO: further processing of packet
+}
+
+void handleUdpPackets()
+{
+    while (const auto &packet = udpListener.poll())
+        handleUdpPacket(*packet);
+}
+```