doc: add 'architecture' and 'settings' sections to the README file

README

@@ -16,6 +16,59 @@ It's then required to get the database configured (./manage.py syncdb); by
 default it will create a db.sqlite3 file.
 
 
+Architecture
+------------
+
+Combo manages content as a series of pages (objects of type 'Page'), that can
+be sorted ('order' attribute) and hierarchically ordered ('parent' attribute).
+
+Every pages have a title and a "slug", that is used as the page URL; a page
+with 'index' as its slug will be served as the index page of the site.
+
+The pages are set to use a template; the templates are defined in the settings
+file (COMBO_PUBLIC_TEMPLATES) and are made of a name, a template file, and a
+serie of "placeholders", identifying locations in the page (main content, side
+bar, footer...).
+
+Example:
+
+    'standard': {
+        'name': 'Standard',
+        'template': 'combo/page_template.html',
+        'placeholders': {
+            'content': {
+                'name': 'Content',
+            },
+            'footer': {
+                'name': 'Footer',
+                'acquired': True,
+            },
+        }
+    }
+
+The content of a page is defined as a serie of cells, of which there are
+various types (they are all subclasses of CellBase); each cell is also
+associated with a placeholder ('placeholder' attribute) and its order within
+('order' attribute).
+
+A placeholder can be marked as 'acquired' (see "footer" in the example above),
+this makes its cells automatically inherited from a parent page; the presence
+of a cell of type 'Unlock Marker' will stop this acquisition mechanism, this
+makes it possible to have a different content for those placeholders in
+specific pages or sections have the site.
+
+  Note: in the case of placeholder acquisition the site index page will be
+  inserted as the top of the page hierarchy.
+
+
+Settings
+--------
+
+Default settings are loaded from settings.py, they can be overloaded by a
+local_settings.py file set in the same directory, or by a file referenced
+in the COMBO_SETTINGS_FILE environment variable.
+
+
 License
 -------