Update example web application

[amitsaha]

The example web application should be updated with the following:

[] Add clearer install and connfiguration instructions (may be docker and docker compose based setup)
[] Explain the Freelancer.com SDK usage
[] Add tests
[] Link to it from docs

Look at https://www.twilio.com/docs/tutorials/account-verification-python-flask#building-blocks for inspiration about how to document the sample application.