Update README.md

Added an empty "how-to-test" section
Merge branch 'master' of github.com:oldfartdeveloper/paypal-ipn-forwarder
Merge pull request #36 from dostapenko/testing

testing works!
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.
Now tracking tmp/ instead of just tmp
Moved 'PaypalIpnForwarder' to be included instead of being a repeated preamble
(m) minor code indenting typo; no logic change
Merge pull request #37 from oldfartdeveloper/test_integration

Branch test_integration now becoming branch master
Merge branch 'master' of github.com:hedgeyedev/paypal-ipn-forwarder