Added layer diagram and started writing on an architecture description page.

dddsample/src/site/apt/architecture.apt

+  ------------
+  Layering
+  ------------
+
+  The sample application is layered as illustrated by this picture:
+
+[images/layers.png]
+
+  There are three vertical layers: interfaces, application and domain, each supported by different kinds of infrastructure.
+
+  * Interfaces
+
+	This layer holds everythng that interacts with other systems, such as web services, RMI interfaces
+	or web applications. It handles interpretation, validation and translation of incoming data. It also
+	handles serialization of outgoing data, such as HTML or XML across HTTP to web browsers or web service
+	clients, or DTO classes and distributed facade interfaces for remote Java clients.
+
+  * Application
+
+    The application layer is responsible for defining the use cases of the application, operations that can
+    are interface-independent and can be synchronous or message-driven. This layer is well suited for spanning
+    transactions, high-level logging and security.
+
+    The application layer is thin in terms of domain logic - it merely coordinates the domain layer objects
+    perform the actual work.
+
+  * Domain
+
+	The domain layer is the heart of the software, and this is where the interesting stuff happens.
+
+    Everything below the domain package: domain service interfaces (and in some cases their implementations),
+    repository interfaces (but not the implementations!) and the domain model with each aggregate in its own
+    subpackage.
+
+
+	Infrastructure
+
+	In adition to the three vertical layers, there's also the infrastructure. As the the picture shows, it supports
+	all of the three layers in different ways, facilitating communication between the layers.
+
+    Hibernate mapping metadata files, Spring context definition files, logging configuration.
+    All external libraries,