Update README.md with POST endpoint example
Update README.md with POST endpoint example

README.md is updated with an example explaining on
how to pass data to a POST endpoint in the request's
body and how to retrieve it later in the endpoints.
PinZhang/slim-test-helpers,manhtai/slim-test-helpers,masterzen/slim-test-helpers,there4/slim-test-helpers,hochanh/slim-test-helpers