Small doc update, add link to relnotes

I noticed we have no way easy way to get to the release notes.  I
updated the front-page text to be a simpler introduction/overview and
provided links to the relnotes and source directly.

Change-Id: I5e339baf2921752ca3d409d82e0cbfc856ead1f8

doc/source/index.rst

@@ -1,15 +1,37 @@
 Diskimage-builder Documentation
 ===============================
 
-Diskimage-builder is a tool for building disk images, file system images and
-ramdisk images.
+``diskimage-builder`` is a tool for automatically building customized
+operating-system images for use in clouds and other environments.
 
-Why?
+It includes support for building images based on many major
+distributions and can produce cloud-images in all common formats
+(``qcow2``, ``vhd``, ``raw``, etc), bare metal file-system images and
+ram-disk images.  These images are composed from the many included
+`elements`; ``diskimage-builder`` acts as a framework to easily add
+your own elements for even further customization.
+
+``diskimage-builder`` is used extensively by the `TripleO project
+<https://wiki.openstack.org/wiki/TripleO>`__ and within `OpenStack
+Infrastructure <http://docs.openstack.org/infra/system-config/>`__.
+
+Code
 ----
 
-Automation: While users and operators can manually script or put together
-ramdisks and disk images, mature automation makes customisation and testing
-easier.
+Release notes for the latest and previous versions are available at:
+
+ * `http://docs.openstack.org/releasenotes/diskimage-builder/
+   <http://docs.openstack.org/releasenotes/diskimage-builder/>`__
+
+
+The code is available at:
+
+ * `https://git.openstack.org/cgit/openstack/diskimage-builder/
+   <https://git.openstack.org/cgit/openstack/diskimage-builder/>`__
+
+
+Table of Contents
+-----------------
 
 .. toctree::
    :maxdepth: 2