Adding section to installation guide about configuring emacs nxml mode.

docs/source/usersguide/install.rst

@@ -179,3 +179,27 @@ Alternatively, you could run from any directory:
 
 Note that in the latter case, any output files will be placed in the present
 working directory which may be different from ``/home/username/somemodel``.
+
+-----------------------------------------------------
+Configuring Input Validation with GNU Emacs nXML mode
+-----------------------------------------------------
+
+The `GNU Emacs`_ text editor has a built-in mode that extends functionality for
+editing XML files. One of the features in nXML mode is the ability to perform
+real-time `validation`_ of XML files against a `RELAX NG`_ schema. The OpenMC
+source contains RELAX NG schemas for each type of user input file. In order for
+nXML mode to know about these schemas, you need to tell emacs where to find a
+"locating files" description. Adding the following lines to your ``~/.emacs``
+file will enable real-time validation of XML input files:
+
+.. code-block:: common-lisp
+
+    (require 'rng-loc)
+    (add-to-list 'rng-schema-locating-files "~/openmc/schemas.xml")
+
+Make sure to replace the last string on the second line with the path to the
+schemas.xml file in your own OpenMC source directory.
+
+.. _GNU Emacs: http://www.gnu.org/software/emacs/
+.. _validation: http://en.wikipedia.org/wiki/XML_validation
+.. _RELAX NG: http://relaxng.org/