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
Why do we all need our own development GitHub apps? What if we had one development repo that someone creates, puts credentials on Heroku staging under `DEV_GITHUB` variables to distinguish from staging, and then pulled them from staging during `bin/setup`?

```bash
# Set up AWS credentials for development environment
if ! grep -qs 'GITHUB' .env; then
  heroku config --remote staging | grep "DEV_GITHUB" >> .env
  find_this="DEV_GITHUB"
  replace_with="GITHUB"
  ag -l $find_this .env | xargs sed -i -e '' "s/$find_this/$replace_with/g"
fi
```