Added note to docs/add_ons.txt about reading the source code.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@3251 bcc190cf-cafb-0310-a4f2-bffc1f526a37

docs/add_ons.txt

@@ -128,6 +128,8 @@ A collection of template filters that implement these common markup languages:
     * Markdown
     * ReST (ReStructured Text)
 
+For documentation, read the source code in django/contrib/markup/templatetags/markup.py.
+
 redirects
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