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
@gylaz I asked Adarsh to add the step describing setting up a "private" Stripe plan because I ran into failures locally before creating the plan. Now that I think about it I guess the interval and amount information isn't necessary.