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
This would work for internal thoughtbot developers looking to set up the app, but not external users who won't have access to the staging environment variables.

If we are not gearing this documentation for external users, this totally makes sense, but my understanding is that is not the case.