doc: add README.md to the example

Author: trombik

examples/default/README.md

@@ -0,0 +1,13 @@
+## What the example does
+
+* Initializes the driver
+* Configure a pin as input
+* Registers an interrupt that logs port value of the INT pin
+* Blinks, or toggles, P10 output
+
+## Wiring
+
+Connect an LED with an appropriate register to P10.
+
+Connect a switch that toggle its voltage HIGH or LOW to INT GPIO. The INT GPIO
+can be set in `menuconfig`. See the defaults in `Kconfig.projbuild`.