Reference the modbus standard in README

[janiversen]

A fair part of the issues we see, are due to people have not read the modbus standard, lets add it to README in the beginning, so we can reference our own README.

Link to standard: https://modbus.org/docs/Modbus_Application_Protocol_V1_1b3.pdf
but I think copying the document is better.

Versions

• Python:
• OS:
• Pymodbus:
• Modbus Hardware (if used):

Pymodbus Specific

• Server: tcp/rtu/ascii - sync/async
• Client: tcp/rtu/ascii - sync/async

Description

What were you trying, what has happened, what went wrong, and what did you expect?

Code and Logs

# code and logs here.

# please use the following to format logs when posting them here
import pymodbus
pymodbus.pymodbus_apply_logging_config()