Added link to 'batteries included' explanation in docs/add_ons.txt

git-svn-id: http://code.djangoproject.com/svn/django/trunk@3509 bcc190cf-cafb-0310-a4f2-bffc1f526a37

docs/add_ons.txt

@@ -2,12 +2,15 @@
 The "contrib" add-ons
 =====================
 
-Django aims to follow Python's "batteries included" philosophy. It ships with a
-variety of extra, optional tools that solve common Web-development problems.
+Django aims to follow Python's `"batteries included" philosophy`_. It ships
+with a variety of extra, optional tools that solve common Web-development
+problems.
 
 This code lives in ``django/contrib`` in the Django distribution. Here's a
 rundown of the packages in ``contrib``:
 
+.. _"batteries included" philosophy: http://docs.python.org/tut/node12.html#batteries-included
+
 admin
 =====