Add instructions to source tutorial setup script (#63)

tutorial_basics.rst

@@ -44,7 +44,18 @@ variable, source the spack setup script.  Then add Spack to your path.
 
   $ . share/spack/setup-env.sh
 
-You're good to go!
+Finally, if you are running in an Amazon Cloud9 instance (which you
+likely are for this tutorial), you will need to run one additional setup
+script:
+
+.. code-block:: console
+
+  $ . share/spack/setup-tutorial-env.sh
+
+You should see a lot of output. This is important, as it will increase
+your available disk space and install some needed system software.
+
+Once you've sourced these scripts, you're good to go!
 
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 What is in Spack?