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
@adarsh yes, thanks. Upon reflection I think lines 34-35 can be removed, per my comment below.