Added note about exporting SYMFONY_ENV

Author: jasonbouffard

cookbook/deployment/tools.rst

@@ -106,6 +106,12 @@ 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
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~