Warn against using some characters in .flux.env

docs/installation/install_stackspin.rst

@@ -21,8 +21,10 @@ Configuration
 
 Copy the file ``install/.flux.env.example`` to your cluster dir
 ``clusters/stackspin.example.org/.flux.env``. This file contains the last bit of
-information you need to configure.  Make sure not to put any quotes around your
-values, because that can make the resulting yaml files invalid.
+information you need to configure. Make sure not to put any quotes around your
+values, because that can make the resulting yaml files invalid. Also, using a
+dollar sign ``$`` or double quote ``"`` may lead to problems, so please avoid
+using those characters in the values.
 
 Cluster information
 ~~~~~~~~~~~~~~~~~~~