Create app.json for Heroku Button

"app.json" is needed for Heroku Button.

The error message about this from Heroku is follwoing;

> No valid app.json file found at: https://github.com/kaosf/env-var-heroku
>
>     No app.json located in the repo URL provided. Are you sure the URL
>     is correct and that an app.json file exists in the project root
>     directory?