docs: add DatabaseTask example

docs/extending.rst

@@ -12,6 +12,36 @@ Extending Task Class
 decorator accept an optional ``task_class`` argument.
 You can pass a subclass of :class:`~kuyruk.Task` to change it's behavior.
 
+Example:
+
+.. code-block:: python
+
+    from kuyruk import Kuyruk, Task
+    from myapp.orm import Session
+
+    kuyruk = Kuyruk()
+
+    class DatabaseTask(Task):
+        """Open a new database session before running tasks
+        and close on finish."""
+
+        def setup(self):
+            self.connect_signal(task_prerun, self.open_session)
+            self.connect_signal(task_postrun, self.close_session)
+
+        def open_session(self, *args, **kwargs):
+            self.session = Session()
+
+        def close_session(self, *args, **kwargs):
+            self.session.close()
+
+
+    @kuyruk.task(task_class=DatabaseTask)
+    def a_task_with_a_session(some_arg):
+        session = self.session
+        # Work with the session...
+        session.commit()
+
 
 Extending Worker Class
 ----------------------