Upgrade README with a minimum example code and library description.

README.md

@@ -5,9 +5,51 @@ Arduino_Portenta_OTA
 [![Arduino Lint](https://github.com/arduino-libraries/Arduino_Portenta_OTA/workflows/Arduino%20Lint/badge.svg)](https://github.com/arduino-libraries/Arduino_Portenta_OTA/actions?workflow=Arduino+Lint)
 [![Spell Check](https://github.com/arduino-libraries/Arduino_Portenta_OTA/workflows/Spell%20Check/badge.svg)](https://github.com/arduino-libraries/Arduino_Portenta_OTA/actions?workflow=Spell+Check)
 
-OTA on the Arduino Portenta.
+This library allows to perform OTA (Over-The-Air) firmware update for the Arduino Portenta H7. OTA binaries are downloaded via WiFi and stored on a SD card or on the Portenta H7's QSPI flash storage. Next all information relevant to the firmware update is stored in non-volatile memory. After a reset the Portenta H7 bootloader is accessing this information and uses it for performing the firmware update.
 
 ### Example
 ```C++
-/* TODO */
+#include <Arduino_Portenta_OTA.h>
+#include <WiFi.h>
+#include "arduino_secrets.h"
+/* ... */
+void setup()
+{
+  if (WiFi.status() == WL_NO_SHIELD)
+    return;
+
+  int status = WL_IDLE_STATUS;
+  while (status != WL_CONNECTED)
+  {
+    status = WiFi.begin(SSID, PASS);
+    delay(10000);
+  }
+
+  Arduino_Portenta_OTA_QSPI ota(QSPI_FLASH_FATFS_MBR, 2);
+  Arduino_Portenta_OTA::Error ota_err = Arduino_Portenta_OTA::Error::None;
+
+  if (!ota.isOtaCapable())
+    return;
+
+  if ((ota_err = ota.begin()) != Arduino_Portenta_OTA::Error::None)
+    return;
+
+  int const ota_download = ota.download("http://downloads.arduino.cc/ota/OTA_Usage_Portenta.ino.PORTENTA_H7_M7.ota", false /* is_https */);
+  if (ota_download <= 0)
+    return;
+
+  int const ota_decompress = ota.decompress();
+  if (ota_decompress < 0)
+    return;
+
+  if ((ota_err = ota.update()) != Arduino_Portenta_OTA::Error::None)
+    return;
+
+  ota.reset();
+}
+
+void loop()
+{
+
+}
 ```