add an example

examples/button.ml

@@ -0,0 +1,22 @@
+open Gpio3
+
+(* Set up the pins the following way: *)
+(* pin 3 (pull-up) -> btn -> resistor -> gnd *)
+
+let setup () =
+  Gpio3.setup ();
+  pin_mode GPIO3 IN;
+  pull_up_dn_control GPIO3 UP
+
+let rec loop () =
+  (match digital_read GPIO3 with
+   | LOW -> print_endline "-- LOW"
+   | HIGH -> print_endline "++ HIGH");
+  Unix.sleepf 0.5;
+  loop ()
+
+
+let _ =
+  setup ();
+  loop ();
+