[#523] Add example

doc/extensions.rst

@@ -75,6 +75,32 @@ stable, backward-compatible and with clear deprecation guidelines as
 development of CKAN core progresses.  This interface is available in
 :doc:`toolkit`.
 
+
+Example Extension
+-----------------
+
+::
+    # Example Extension
+    # This extension adds a new template helper function `hello_world` when
+    # enabled templates can `{{ h.hello_world() }}` to add this html snippet.
+
+    import ckan.plugins as p
+
+    class HelloWorldPlugin(p.SingletonPlugin):
+
+        p.implements(p.ITemplateHelpers)
+
+        @staticmethod
+        def hello_world():
+            # This is our simple helper function.
+            html = '<span>Hello World</span>'
+            return p.toolkit.literal(html)
+
+        def get_helpers(self):
+            # This method is defined in the ITemplateHelpers interface and
+            # is used to return a dict of named helper functions.
+            return {'hello_world': hello_world}
+
 Guidelines for writing extensions:
 ----------------------------------