Add comment to rig.h about not modifying callbacks structure

include/hamlib/rig.h

@@ -2853,6 +2853,7 @@ typedef int (*spectrum_cb_t)(RIG *,
  * \sa rig_set_freq_callback(), rig_set_mode_callback(), rig_set_vfo_callback(),
  *     rig_set_ptt_callback(), rig_set_dcd_callback()
  */
+// Do NOT add/remove from this structure -- it will break DLL backwards compatiblity
 struct rig_callbacks {
     freq_cb_t freq_event;   /*!< Frequency change event */
     rig_ptr_t freq_arg;     /*!< Frequency change argument */