ENH: Updates to documentation (Chris)

doc/Doxygen/README

+Building the Doxygen help for OpenFOAM
+======================================
+
+The HTML documentation of OpenFOAM can be built using the Doxygen
+software.  Images in the documentation are generated using "dot", a
+program within the graphviz software package.  To build the
+documentation on the Ubuntu Linux system, for example, the user should
+install both doxygen and graphviz packages, e.g. by typing in a
+terminal window:
+
+    sudo apt-get install doxygen graphviz
+
+The Doxygen documentation will build automatically for a user that is
+configured to run OpenFOAM, i.e. has environment variables such as
+$WM_PROJECT_DIR set.  The user must also ensure they have write
+permission for the directory that Doxygen writes files to.
+
+The Doygen configuration file, Doxyfile, in the
+$WM_PROJECT_DIR/doc/Doxygen directory is configured to work with
+Doxygen versions 1.6.3-1.8.1.
+
+
+**Note**:
+    For versions of Doxygen below 1.7, the final <div> in the
+    _Header file (line 37) must be deleted before running Doxygen to
+    produce the correctly formatted HTML files.