Added pre-flight note about hardware watchdog

firmware/Pre-flight.md

@@ -6,6 +6,8 @@
   - `APRS_ENABLE` is set if APRS is to be used on the flight
   - `APRS_USE_GEOFENCE` is set
   - `DEBUG_USE_INTWATCHDOG` can be disabled during flight. This saves some power
+  - Check interrupts are regular enough to kick the hardware
+    watchdog.
 
 - In `watchdog.h`
   - Set the various `MAXIDLE` values to values determined during