Adding Postman Button to Project README

[kinlane]

Great resource! I see that you mention Postman for testing on project README, and have a collection available for getting up and running (https://github.com/CMSgov/dpc-app/blob/master/src/test/EndToEndRequestTest.postman_collection.json) -- great start.

I also recommend publishing a "Run in Postman" button as part of the README, which would allow people to quickly import the Postman collection + testing into their own Postman client in a single click, further reducing friction when on-boarding.

Thanks for the hard work and let me know if you need assistance in doing. I'd make a PR but you should drive from your collection to maintain control over the collection.