instructions for setting SYMFONY_ENV on Heroku for smooth deploys
capitalize section title as per feedback
remove first person as per feedback
feature #4591 Instructions for setting SYMFONY_ENV on Heroku (dzuelke)

This PR was merged into the 2.3 branch.

Discussion
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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?

Commits
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576bbfb remove first person as per feedback
579fdd6 capitalize section title as per feedback
5f987ff instructions for setting SYMFONY_ENV on Heroku for smooth deploys