Replaced the Doxygen files in the doc folder (issue #373)

doc/readme.txt

@@ -0,0 +1,61 @@
+Generating Documentation for OWA-EPANET 2.2
+===========================================
+
+You must have Doxygen (http://www.doxygen.nl/)installed on your machine to generate
+documentation for OWA-EPANET's API (aka Toolkit). Assuming this is the case, open a
+terminal window, navigate to the project's 'doc' directory and issue the command
+'doxygen' to generate documentation in the following formats:
+
+- HTML documentation will be placed in the 'html' sub-directory. Launch 'index.html'
+  to view it in a web browser.
+
+- Latex documentation will be placed in the 'latex' sub-directory. Assuming you
+  have a TeX system, such as MikTex (https://miktex.org), installed on your machine
+  you can generate a PDF of the documentation by issuing the 'make pdf' command from
+  within the 'latex' directory. The resulting pdf file will be named 'refman.pdf'.
+
+To generate a Windows compiled HTML Help file you must have Microsoft's HTML Help Workshop
+(https://www.microsoft.com/en-us/download/details.aspx?id=21138) installed. In this case
+you need to first edit the Doxygen configuration file 'Doxyfile' as follows:
+
+1. Change the 'GENERATE_HTMLHELP' setting to 'YES'.
+
+2. Enter the location where the Help Workshop system was installed next to the
+   'HHC_LOCATION' setting.
+
+After running 'doxygen' again the resulting Help file named 'owa-epanet.chm' will
+appear in the 'html' sub-directory.
+
+Doxygen uses the special comments placed in the project's 'epanet2_2.h' and
+'epanet2_enums.h' header files to document EPANET's API. It also uses supplementary
+material contained in the following files of the project's 'doc' folder to generate
+additional pages of documentation:
+
+main.dox - generates the Overview pages.
+
+toolkit-usage.dox: generates the Toolkit Usage page.
+
+toolkit-examples.dox : generates the Toolkit Examples pages.
+
+modules.dox: generates the Reference section of the document consisting of several
+             module pages that describe Toolkit functions by group, enumerated
+             constants, file descriptions, error codes, property units, and output
+             file format.
+             
+output-format.dox: generates the pages that describe the format used in different
+                   sections of the output file. 
+
+Finally, a group of special Doxygen files are used to customize the format of the
+generated documentation. These include the following:
+
+Doxyfile - the main Doxygen configuration file
+
+DoxygenLayout.xml - replaces the title "Modules" with "Reference" and hides the
+                    "Files" section in the tree view pane of the document. 
+                    
+extrastylesheet.css - reduces the size of the the h1 heading style.
+
+newfooter.html - replaces the default Doxygen footer in HTML output with a custom one.
+
+header.tex - replaces the standard title page and footer text used in Latex output.
+