Add heroku deploy

README.md

 # Election Runner
 ## Michigan Flyers Digital and Paper Ballot Hybrid Tally System
 
-[![Deploy to DO](https://www.deploytodo.com/do-btn-blue-ghost.svg)](https://cloud.digitalocean.com/apps/new?repo=https://github.com/michiganflyers/elections/tree/main)
+[![Deploy to Digital Ocean](https://www.deploytodo.com/do-btn-blue-ghost.svg)](https://cloud.digitalocean.com/apps/new?repo=https://github.com/michiganflyers/elections/tree/main)
+[![Deploy to Heroku](https://www.herokucdn.com/deploy/button.svg)](https://heroku.com/deploy?template=https://github.com/michiganflyers/elections/tree/main)

app.json

+{
+	"name": "Michigan Flyers Elections",
+	"description": "Election-handling system for hybrid in-person/online voting.",
+	"repository": "https://github.com/michiganflyers/elections",
+	"stack": "container"
+}

heroku.yml

+build:
+  docker:
+    web: Dockerfile