Add documentation on virtual port

[wlkh]

Describing how to use the functionality from pull request #2

[alexisgaziello]

For the documentation do you prefer that I explain the use of socat in:

• a separate file
• an extension at the end of the README.md file
• a big comment before the class VirtualPort in the python script

In addition I can add some comments detailing each action in the code + more description in the README.md file.