Add app.json and Deploy to Heroku button

_I realize this may not be the best project for a Deploy to Heroku button, so feel free to ignore/close this. I was mostly just playing around to test something out and figured I might as well offer it as a contribution._

This adds an [`app.json` manifest](https://devcenter.heroku.com/articles/app-json-schema) and a [Deploy to Heroku button](https://devcenter.heroku.com/articles/heroku-button) to the readme that simplifies the existing setup instructions.