Additional PwmOut example

Revision:
0:07220dd760cc
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/main.cpp	Thu Jan 19 14:22:09 2017 -0600
@@ -0,0 +1,10 @@
+#include "mbed.h"
+
+PwmOut led(LED2);
+
+int main() {
+    // specify period first, then everything else
+    led.period(4.0f);  // 4 second period
+    led.write(0.50f);  // 50% duty cycle
+    while(1);          // led flashing
+}