Add explanation to readme about how to use the hardward serial.

[RobinLeblond]

Hello, I didn't know how to ask this elsaway, but can you explain how to use it with the hardware serial instead of the sofware one ? All your example look like using the software serial, thanks.

[RealTadango]

Better documentation is a good idea 😊

You can use hardware serial like this:

SPortHub hub(0x12, Serial);

[RobinLeblond]

Wow, easy like that! Thanks

[icebreaker-ch]

Unfortunately, this solution does not work due to several reasons:

1. The constructor of SPortHub with a Serial parameter is hidden in a strange conditional compilation statement:

 #ifdef Serial_
            SPortHub(int physicalId, Serial_& serial);
    #elif HardwareSerial
            SPortHub(int physicalId, HardwareSerial& serial);
    #endif

I think, the different constructors shall be all visible and overloaded.

2. Because of the conversion operator
   Serial_::operator bool()
   in the class
   Serial_
   the constructor used for
   SPortHub(0x12, Serial);
   is again;
   SPortHub(int physicalId, int softwarePin);
   which makes no sense at all, but we get no error message here.