The very first documentation file

Change-Id: Ie13d3eec555bca1a138fb69444aa429308fc9b3b

documentation/book-preface.asciidoc

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+---
+title: Preface
+order: 3
+layout: page
+---
+
+== Preface
+
+This book provides an overview of the Vaadin Framework and covers the most
+important topics that you might encounter while developing applications with it.
+A more detailed documentation of the individual classes, interfaces, and methods
+is given in the Vaadin API Reference.