add app.json

Hi @jeffandersen!

`app.json` is an open manifest we're working on at Heroku for describing web apps and their runtime dependencies. This goinstant app is a perfect candidate for `app.json`, as it requires a custom buildpack and an addon. Are you interested in helping us beta test the build service we're working on for `app.json`?

For more context, check out the work-in-progress schema doc and the node module:

- https://devcenter.heroku.com/articles/app-json-schema?preview=1
- https://github.com/heroku/app.json#readme

cc @ryanbrainard @balansubr @friism @max