add an example about how to use Kconfig file

Signed-off-by: Nicola Lunghi <nicola.lunghi@jci.com>

main/main.c

@@ -22,8 +22,8 @@ void app_main(void)
     ESP_ERROR_CHECK( esp_wifi_set_mode(WIFI_MODE_STA) );
     wifi_config_t sta_config = {
         .sta = {
-            .ssid = "access_point_name",
-            .password = "password",
+            .ssid = CONFIG_ESP_WIFI_SSID,
+            .password = CONFIG_ESP_WIFI_PASSWORD,
             .bssid_set = false
         }
     };