add compile doku and fix compile warnings

examples/osd/arduino-merkurboard/README.md

@@ -1,6 +1,8 @@
 Arduino compatibility example
 =============================
 
+make clean TARGET=osd-merkur-256 flash
+
 This example shows that it is now possible to re-use arduino sketches in
 Contiki. This example documents the necessary magic. Arduino specifies
 two routines, `setup` and `loop`. Before `setup` is called, the

examples/osd/arduino-merkurboard/sketch.pde

@@ -29,9 +29,11 @@ void setup (void)
     led_status=0;
     // init coap resourcen
     rest_init_engine ();
+    #pragma GCC diagnostic ignored "-Wwrite-strings"
     rest_activate_resource (&res_led, "s/led");
     rest_activate_resource (&res_battery, "s/battery");
     rest_activate_resource (&res_cputemp, "s/cputemp");
+    #pragma GCC diagnostic pop
     
  //   NETSTACK_MAC.off(1);
     mcu_sleep_set(128);