Add warning to docs re: client assertion of DTR causing board reset

[c0d3rx]

Some modbus client (e.g pymodbus) assert dtr before transmitting, this lead to reset arduino to bootloader ( i spent one day around the problem on arduino-nano used as modbus-slave)
Resolved disabling reset with a 120 ohm resistor from 5v to rst to disable dtr reset.
After that, my program works flawless
Not an issue but i think would be nice to warn user if strange behaviour happen.
Thank you for sharing your work!