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
Should we be mentioning `3000` or just `port_number`? Doesn't foreman default to something other than `3000`?