Instructions for setting SYMFONY_ENV on Heroku

Much smoother sailing with that step included :)

On a related note, I was going to add general advice on using `SYMFONY_ENV`, as it is currently very under-documented (just one aside mention at http://symfony.com/doc/current/cookbook/deployment/tools.html#c-update-your-vendors), but I wasn't sure where to do this. Maybe in `cookbook/configuration/environments.rst` and a few other places?