[Docs] Docs for Heroku with examples of Procfile and env vars

[hannelita]

Acceptance

• [ ] Write proper docs for Heroku deploy (describe Procfile and ENV vars configuration)

[shevron]

The docs now contain more information about configuration using .env files. I'm not sure what a Procfile is (something Heroku specific?). If we want to add a "deploying on Heroku" guide to the docs, I'm all for that (but I don't have that knowledge). If we don't, I suggest we close this.

/cc @rufuspollock