doc: Add filter cookbook (fix #966)

doc/cookbook/custom-filter.rst

@@ -3,4 +3,38 @@
 Create a custom filter
 ======================
 
-.. todo:: this chapter
+Create custom :ref:`template filters <guide-filters>` to change the way
+variables are rendered in your templates. By following some simple rules,
+Zotonic will automatically find the filter for you:
+
+1. Create a file in the :ref:`module-directory-filters` directory of
+   your site or module.
+2. Prepend the filter filename with ``filter_``.
+3. Export a function with the name of your filter that corresponds to the
+   filename.
+
+So, let’s say you need to sort a list of items and remove duplicate values from
+it:
+
+.. code-block:: erlang
+    :caption: mod_yourmodule/filters/filter_uniquesort.erl
+
+    -module(filter_uniquesort).
+    -export([uniquesort/2]).
+    -include("zotonic.hrl").
+
+    uniquesort(List, _Context) ->
+        lists:usort(List).
+
+The custom ``uniquesort`` filter is then available in your templates:
+
+.. code-block:: django
+
+    {% for thing in list_of_things|unique %}
+        {{ thing }} is now sorted and unique!
+    {% endfor %}
+
+.. seealso::
+
+    * :ref:`guide-templates` guide
+    * :ref:`filters` reference

doc/developer-guide/modules.rst

@@ -220,6 +220,8 @@ The following naming conventions for templates are used:
 
 .. seealso:: :ref:`guide-templates`
 
+.. _module-directory-filters:
+
 filters/
 ........