Update README.md

Added an empty "how-to-test" section
Merge branch 'master' of github.com:oldfartdeveloper/paypal-ipn-forwarder
Merge branch 'master' of github.com:hedgeyedev/paypal-ipn-forwarder
This is the final work that Dmitri and Scott did together before the end of his internship.
Merge branch 'master' of github.com:hedgeyedev/paypal-ipn-forwarder
Forcing test to believe host machine is OSX even though running on Linux server