Add attribution links in README

This is in response to the tweet you sent to [@busyconf](https://twitter.com/busyconf).

https://twitter.com/danrabinowitz/status/503060668866498560

We're glad and flattered that the receipt inspired you. We just ask that
you link the README to our website. Thanks! -Ryan (BusyConf Founder)

P.S. I like the use of css to achieve a similar affect. We chose to
simulate more of a line printer on the backend, which has some
advantages to what we have to output (especially for ensuring email
receipts look okay), but it has its cons too.