Added instructions for modifying XML input in devguide.

docs/source/devguide/xml-fortran.rst

@@ -4,3 +4,37 @@
 xml-fortran Input Parsing
 =========================
 
+OpenMC relies on the xml-fortran package for reading and intrepreting the XML
+input files for geometry, materials, settings, tallies, etc. The use of an XML
+format makes writing input files considerably more flexible than would otherwise
+be possible.
+
+With the xml-fortran package, extending the user input files to include new tags
+is fairly straightforward. A "template" file exists for each diferent type of
+input file that tells xml-fortran what to expect in a file. These template files
+can be found in the src/templates directory. The steps for modifying/adding
+input are as follows:
+
+1. Add a ``<variable>``` tag to the desired template file,
+e.g. src/templates/geometry_t.xml. See the `xml-fortran documentation`_ for a
+description of the acceptable fields.
+
+2. In the input_xml module, any input given in your new tag will be read
+automatically through a call to, e.g. read_xml_file_geometry_t. Whatever
+variable name you specified should have the data available.
+
+3. Add code in the appropriate subroutine to check the variable for any possible
+errors.
+
+4. Add a variable in OpenMC to copy the temporary variable into if there are no
+errors.
+
+A set of `RELAX NG`_ schemata exists that enables real-time validation of input
+files when using the GNU Emacs text editor. You should also modify the RELAX NG
+schema for the template you changed (e.g. src/templates/geometry.rnc) so that
+those who use Emacs can confirm whether their input is valid before they
+run. You will need to be familiar with RELAX NG `compact syntax`_.
+
+.. _xml-fortran documentation: http://xml-fortran.sourceforge.net/documentation.html
+.. _RELAX NG: http://relaxng.org/
+.. _compact syntax: http://relaxng.org/compact-tutorial-20030326.html