Add modbus standard to README. (pymodbus-dev#1308)

Add modbus standard doc to README.

Author: janiversen

README.rst

@@ -34,6 +34,7 @@ Summary
 
 Pymodbus is a full Modbus protocol implementation using a synchronous or asynchronous (using asyncio) core.
 
+The modbus protocol documentation can be found `here <../_static/Modbus_Application_Protocol_V1_1b3.pdf>`
 Supported modbus communication modes: tcp, rtu-over-tcp, udp, serial, tls
 
 Pymodbus can be used without any third party dependencies (aside from pyserial) and is a very lightweight project.