boards/kakute-f7-mini-v3: document MAVLink serial output

boards/kakute-f7-mini-v3/README.md

@@ -42,8 +42,9 @@ dfu-util --alt 0 --dfuse-address 0x08000000:leave \
 | `GND` | `G` |
 | `TX` | `R1` |
 
-Open the serial device at 115200 baud, 8N1:
+The transmitter emits unsigned MAVLink 2 at 115200 baud, 8N1. It is binary
+data, not terminal text. Read it with the python decoder:
 
 ```sh
-picocom --baud 115200 --databits 8 --parity n --stopbits 1 $serial_dev_path
+./scripts/read-mavlink.py $serial_device_path
 ```