Peter M. Groen / libmodbus

Add macros in libmodbus.txt and:
- modbus_get_byte_from_bits.txt
- modbus_get_float.txt
- modbus_reply.txt
- modbus_reply_exception.txt
- modbus_set_bits_from_byte.txt
- modbus_set_bits_from_bytes.txt
- modbus_set_float.txt