Adding documentation in Readme and an advance example using SoftwareSerial

[sanujkul]

Updates one are:

1. Updated the readme file with the documentation of the functions available and how to use them.
2. Added an advance example that uses SoftwareSerial object as an output stream for the microcontrollers that only have one Serial port and use SoftwareSerial to create more ports.

SoftwareSerial mySerial(10, 11); // RX, TX  
mySerial.begin(9600);  
Debug.setDebugOutputStream(&mySerial);

[TravisBuddy]

Travis tests have failed

Hey @sanujkul,
Please read the following log in order to understand the failure reason.
It'll be awesome if you fix what's wrong and commit the changes.

TravisBuddy Request Identifier: 4623bb30-606d-11ea-afc8-ddb27bdee665