Document deployment to Heroku

[paulcsmith]

What ENV vars are necessary, how to get Google setup, Mandrill, what mix tasks to run etc.

[tjmw]

I can take a look at this - I had to go through most of these questions when setting up review apps #293. I'll add instructions to DEPLOYMENT.md at the root of the repo.

[tjmw]

Another idea - how about providing a "Deploy to Heroku" button as described here. We already have the app.json manifest for review app configuration. There's one tb specific piece in there (the redirector config) but hopefully we can work around that.

[paulcsmith]

@tjmw Great idea! I'm working on this today. It looks like we can provide default ENV vars in the deploy button. I'll try some stuff out and see if I can make it work for the button and our review apps