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
Do we want `HOST` in here? I think we usually generate it in `bin/setup` and put it in `.foreman`:

https://github.com/thoughtbot/suspenders/blob/master/templates/bin_setup.erb#L28-L30

We randomly sample one of a few port numbers:

https://github.com/thoughtbot/suspenders/blob/master/lib/suspenders/app_builder.rb#L387-L389