Added a design decisions doc

docs/source/design_decisions.rst

+================
+Design decisions
+================
+
+Oscar's is designed to be open to customisation wherever possible.  There are two
+main ways in which this is acheived.
+
+Dynamic importing of classes
+----------------------------
+
+Within oscar itself, classes are imported using a special ``import_module`` function
+which determines which app to load the specified classes from.  Sample usage is::
+
+    import_module('product.models', ['Item', 'ItemClass'], locals())
+    
+This will load the ``Item`` and ``ItemClass` classes into the local namespace.  It is
+a replacement for the usual::
+
+    from oscar.product.models import Item, ItemClass
+    
+The ``import_module`` looks through your ``INSTALLED_APPS`` for a matching module to
+the one specified and will load the classes from there.  If the matching module is
+not from oscar's core, then it will also fall back to the equivalent module if the
+class cannot be found.
+
+This structure enables a project to create a local ``product.models`` module and 
+subclass and extend the core models from ``oscar.app.product.models``.  When Oscar
+tries to load the ``Item`` class, it will load the one from your local project.
+
+Class-based views
+-----------------
+
+All views within oscar's core are class-based, which allows the above dynamic class
+loading to be used within ``urls.py`` modules, so that core views can be subclassed
+within projects to extend and customised them.
+

docs/source/index.rst

 
    introduction
    ecommerce_domain
+   design_decisions
    getting_started
    components
    web_services