improve documentation for 0.4 release

Author: wachterjohannes

components.rst

@@ -0,0 +1,35 @@
+Components
+==========
+The library consists of three main parts.
+
+TaskScheduler
+-------------
+The ``TaskScheduler`` stores tasks and schedules Executions. Foreach tasks there
+exists always on execution in the status ``planned``, ``running``. If the task
+will only one time it only get one execution at all. For recurring tasks the
+execution will always generated for the next run time.
+
+Each task consists of:
+
+- ``uuid``: unique identifier
+- ``handlerClass``: class-name of the handler which will be executed for this
+  task
+- ``workload``: the workload will be passed to the handler
+- ``intervall``:
+- ``firstExecution``:
+- ``lastExecution``:
+
+TaskRunner
+----------
+The ``TaskRunner`` takes the ``planned`` executions and executes the ``Handler``
+with the workload of the task.
+
+Handler
+-------
+The ``Handler`` implements the domain-logic for a task. The library implements
+a Factory which uses Reflection to create a new instance for the
+``handlerClass`` of the task.
+
+.. note::
+
+    The :doc:`symfony` uses tagged-services to find available ``Handler``.