Update custom theme templates, document default

The documentation referenced an "index.html" template which appears to
be no longer used. Update the documentation so that it talks about the
correct templates, album.html and album_list.html.

Also, add documentation about the default theme, talking about what it
contains and what it doesn't.

docs/themes.rst

@@ -2,8 +2,7 @@
  Themes
 ========
 
-Gallery pages are created from a `Jinja2`_ template ``index.html`` that must
-be located in ``THEME_DIR/templates``.
+Sigal themes are created using the `Jinja2`_ templating system.
 
 .. _Jinja2: http://jinja.pocoo.org/docs/
 
@@ -30,6 +29,27 @@ Sigal comes with three themes, located in the ``sigal/themes`` folder:
     `source <http://photoswipe.com/>`__, `demo
     <http://saimon.org/sigal-demo/photoswipe/>`__.
 
+The ``themes`` folder also includes a ``default`` theme. This theme cannot be
+used on its own, instead, it contains Jinja2 templates that are used by the
+bundled themes to enable functionality such as the feeds and ZIP gallery
+plugins, as well as settings such as Google Analytics.
+
+Creating a theme
+~~~~~~~~~~~~~~~~
+
+Sigal requires the following two templates in order to generate a gallery.
+These templates must be located in ``THEME_DIR/templates``:
+
+- ``album_list.html``: Generates the album list, if you have multiple albums
+- ``album.html``: Generates each individual album page
+
+Themes may also include static content such as CSS and JavaScript by placing
+this content in the ``THEME_DIR/static`` folder. You can then access this
+content in your template by using the ``theme.url`` variable.
+
+Custom themes may also include the templates available in the ``default``
+theme.
+
 Variables
 ~~~~~~~~~