Maybe the lack of doc is intentional, but to have Flask/React app on Heroku, you need:
– https://github.com/ddollar/heroku-buildpack-multi
– to run grunt build in a sub-folder (project/static/Gruntfile.js), add a package.json at the root folder. Based on the knowledge that heroku will run ‘npm install’

{
  "dependencies": {
    "grunt": "^0.4.5",
    "grunt-cli": "*"
  },
  "scripts": {
    "postinstall": "npm install -g grunt-cli bower browserify && cd project/static && npm install && cd ../.. && grunt heroku --gruntfile project/static/Gruntfile.js"
  }
}