Add design goal to documentation

Author: EpicWink

docs/src/index.rst

@@ -12,6 +12,11 @@ Typed Python interface to AWS Simple Workflow service
   on-demand)
 * Better execution filtering
 
+The most interesting part is state construction: given an execution history, this
+library can build a full state of the execution and all of its tasks with all details;
+see :meth:`swf_typed.build_state`). The rest of the API simply wraps and closely
+matches AWS's SWF API.
+
 
 See also
 ^^^^^^^^