Clarifies single line for Python 2 and 3

python3_user_transition_guide.rst

@@ -4,6 +4,20 @@ Python 3 transition guide
 Switching to Python 3 can seem like a daunting task, but this guide will
 provide some tips and resources to help make it more straightforward.
 
+Changes for vanilla Python to support both Python 2 and 3
+---------------------------------------------------------
+Vanilla Python code (i.e., typical scientific code)
+does not need much modification to run under *both* Python 2 and Python 3.
+
+For these scripts, including the line below at the top
+of the script makes the script run under both Python 2 and Python 3.
+
+.. code-block:: python
+
+    from __future__ import print_function, division, absolute_import
+
+The next section provides an explanation of what this provides.
+
 Changes you can make today to make the transition easier
 --------------------------------------------------------