minor #37 Document the special %generate(secret)% env var value (javiereguiluz)

This PR was squashed before being merged into the master branch (closes #37).

Discussion
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Document the special %generate(secret)% env var value

Some recipes are starting to use this feature (see https://github.com/symfony/recipes-contrib/pull/5/files) so let's document it.

Commits
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8a103f4 Document the special %generate(secret)% env var value

Author: fabpot

README.rst

@@ -204,6 +204,11 @@ The ``###> your-recipe-name-here ###`` section separators are needed by Symfony
 to detect the contents added by this dependency in case you uninstall it later.
 Don't remove or modify these separators.
 
+.. tip::
+
+    Use ``%generate(secret)%`` as the value of any environment variable to
+    replace it with a cryptographically secure random value of 16 bytes.
+
 ``makefile`` Configurator
 ~~~~~~~~~~~~~~~~~~~~~~~~~