Add docs for not found (404) page

MarkdownSource/docnet.json

@@ -14,7 +14,8 @@
         "Configuration" :
         {
             "Site configuration with the docnet.json file" : "docnetjson.md",
-            "Automatic H2 level ToC entry discovery" : "h2leveldiscovery.md"
+            "Automatic H2 level ToC entry discovery" : "h2leveldiscovery.md",
+			"Not found (404) page" : "notfound.md"
         },
         "Authoring content" : 
         {

MarkdownSource/notfound.md

@@ -0,0 +1,6 @@
+Not found (404) page
+====================
+
+`Docnet` will automatically create a not found (404) page in the root of the destination folder named `404.htm`.
+
+If no `404.md` file is found in the root of the source folder, the file will be generated with default content. If a file is found, the `404.md` will be used to generate the not found (404) page.