[pre-flight] Add reminder to enable rf tx

firmware/Pre-flight.md

@@ -3,6 +3,7 @@
 - In `hw_config.h`
   - `USE_XOSC` is set. Otherwise the symbol timing frequency will be
     somewhat inaccurate.
+  - `RF_TX_ENABLE` is set
   - `APRS_ENABLE` is set if APRS is to be used on the flight
   - `APRS_USE_GEOFENCE` is set
   - `TELEMETRY_USE_GEOFENCE` is set