feature #4182 Added note about exporting SYMFONY_ENV (jpb0104)

This PR was merged into the 2.3 branch.

Discussion
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Added note about exporting SYMFONY_ENV

New PR to replace the mess I made in #4001. Sorry about that. I also applied all the suggestions.

Commits
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15ce0fe Added note about exporting SYMFONY_ENV

cookbook/deployment/tools.rst

@@ -110,6 +110,12 @@ as you normally do:
     ensures that development packages are not installed in the production
     environment.
 
+.. caution::
+
+    If you get a "class not found" error during this step, you may need to
+    run ``export SYMFONY_ENV=prod`` before running this command so that
+    the ``post-install-cmd`` scripts run in the ``prod`` environment.
+
 D) Clear your Symfony Cache
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~