Add cross-ref to section about creating new derived variables

Docs/source/creating_a_problem.rst

@@ -200,7 +200,9 @@ each of these in the main source tree.
    problem which refines a rectangular region (fuel layer) based on
    a density parameter and the H mass fraction.
 
--  ``Problem_Derives.H``, ``Problem_Derive.H``, and ``Problem_Derives.cpp``
+   .. _problem_derives:
+
+-  ``Problem_Derives.H``, ``Problem_Derive.H``, and ``Problem_Derive.cpp``
 
    Together, these provide a mechanism to create derived quantities
    that can be stored in the plotfile. ``Problem_Derives.H``

Docs/source/io.rst

@@ -397,6 +397,8 @@ Derived variables
 problem-specific plotfile variables
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
+See the section on :ref:`Problem_Derives.H <problem_derives>` for more details about defining your own plotfile variables.
+
 +-----------------------------------+---------------------------------------------------+--------------------------------------+
 | variable name                     | description                                       | units                                |
 +===================================+===================================================+======================================+