helper/list.h: add mention to the example in contrib

Without such reference, it could be difficult to find the example.

Change-Id: Ia9ffb06bc1a45446c2c7b53197ab3400e1d8a9e9
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: http://openocd.zylin.com/6281
Tested-by: jenkins
Reviewed-by: Tim Newsome <tim@sifive.com>

src/helper/list.h

@@ -10,6 +10,8 @@
  *   list_prepare_entry;
  * - expand READ_ONCE, WRITE_ONCE, smp_load_acquire, smp_store_release;
  * - make comments compatible with doxygen.
+ *
+ * There is an example of using this file in contrib/list_example.c.
  */
 
 #ifndef OPENOCD_HELPER_LIST_H