Add SDU usage example

libraries/SDU/examples/Usage/Usage.ino

@@ -0,0 +1,73 @@
+/*
+ Usage
+ This example demonstrates how to use the SAMD SDU library to update a 
+ sketch on an Arduino/Genuino Zero, MKRZero or MKR1000 board using an
+ SD card. It prints out the date and time the sketch was compiled at 
+ to both Serial and Serial1.
+
+ Circuit:
+ * Arduino MKRZero board with SD card
+ OR
+ * Arduino/Genuino Zero or MKR1000 board
+ * SD shield or breakout connected with CS pin of 4
+ * SD card
+
+ Non-Arduino/Genuino Zero, MKRZero or MKR1000 board are NOT supported.
+
+ Steps to update sketch via SD card:
+
+ 1) Upload this sketch or another sketch that includes the SDU library
+    via #include <SDU.h>
+
+ 2) Update the sketch as desired. For this example the sketch prints out
+    the compiled date and time so no updates are needed.
+
+ 3) In the IDE select: Sketch -> Export compiled Binary
+
+ 4) Copy the .bin file from the sketch's folder to the SD card and rename
+    the file to UPDATE.bin. Eject the SD card from your PC.
+
+ 5) Insert the SD card into the board, shield or breakout and press the
+    reset button or power cycle the board. The SDU library will then update
+    the sketch on the board with the contents of UPDATE.bin
+
+ created 23 March 2017
+ by Sandeep Mistry
+*/
+
+/*
+ Include the SDU library 
+
+ This will add some code to the sketch before setup() is called
+ to check if an SD card is present and UPDATE.bin exists on the
+ SD card.
+
+ If UPDATE.bin is present, the file is used to update the sketch
+ running on the board. After this UPDATE.bin is deleted from the
+ SD card.
+*/
+#include <SDU.h>
+
+String message;
+
+void setup() {
+  Serial.begin(9600);
+  Serial1.begin(9600);
+
+  // wait a bit
+  delay(1000);
+
+  message += "Sketch compile date and time: ";
+  message += __DATE__;
+  message += " ";
+  message += __TIME__;
+
+  // print out the sketch compile date and time on the serial port
+  Serial.println(message);
+  Serial1.println(message);
+}
+
+void loop() {
+  // add you own code here
+}
+