Revise setup instructions in README.md

* Set up local environment and clarified instructions in `README.md`
* Centralize on localhost:3000, since that's likely the default port
* Clearer GitHub and Stripe instructions

https://trello.com/c/TpFwhtlt/333-replicate-setup-locally-and-update-readme-me
Actually you're right, `5000` is the default port number Foreman runs on.

https://github.com/ddollar/foreman/search?utf8=%E2%9C%93&q=5000

Fixed in 6170f91