Move reference docs => src/reference

This change eliminates the spring-framework-reference subproject, moving
these sources into the root project's own src directory.

This makes sense because the reference docs span all submodules, and
also because api Javadoc is created at the root project level as well.
This means that both api and reference documentation output will now
reside in the root project's 'build' directory. This is more consistent
and easy to discover.

src/reference/docbook/preface.xml

@@ -0,0 +1,39 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<!DOCTYPE preface PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN"
+ "http://www.oasis-open.org/docbook/xml/4.5/docbookx.dtd">
+
+<preface id="preface">
+  <title>Preface</title>
+
+  <para>Developing software applications is hard enough even with good tools
+  and technologies. Implementing applications using platforms which promise
+  everything but turn out to be heavy-weight, hard to control and not very
+  efficient during the development cycle makes it even harder. Spring provides
+  a light-weight solution for building enterprise-ready applications, while
+  still supporting the possibility of using declarative transaction
+  management, remote access to your logic using RMI or web services, and
+  various options for persisting your data to a database. Spring provides a
+  full-featured <link linkend="mvc-introduction">MVC framework</link>, and
+  transparent ways of integrating <link linkend="aop-introduction">AOP</link>
+  into your software.</para>
+
+  <para>Spring could potentially be a one-stop-shop for all your enterprise
+  applications; however, Spring is modular, allowing you to use just those
+  parts of it that you need, without having to bring in the rest. You can use
+  the IoC container, with Struts on top, but you could also choose to use just
+  the <link linkend="orm-hibernate">Hibernate integration code</link> or the
+  <link linkend="jdbc-introduction">JDBC abstraction layer</link></para>
+
+  <para>Spring has been (and continues to be) designed to be non-intrusive,
+  meaning dependencies, from your domain logic code, on the framework itself
+  are generally none. For your integration layer like the data access layer
+  there will of course be some dependencies on the data access technology in
+  use and also on the Spring libraries, but these dependencies should be easy
+  to isolate from the rest of your code base.</para>
+
+  <para>This document provides a reference guide to Spring's features. Since
+  this document is still to be considered very much work-in-progress, if you
+  have any requests or comments, please post them on the user mailing list or
+  on the support forums at <ulink url="http://forum.springsource.org/" />.
+  </para>
+</preface>