README.md: Add document introducing VTK and linking to resources

Port introduction text from "README.html".  Write the rest of the
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README.md

+![VTK - The Visualization Toolkit](vtkBanner.gif)
+
+Introduction
+============
+
+VTK is an open-source software system for image processing, 3D
+graphics, volume rendering and visualization. VTK includes many
+advanced algorithms (e.g., surface reconstruction, implicit modelling,
+decimation) and rendering techniques (e.g., hardware-accelerated
+volume rendering, LOD control).
+
+VTK is used by academicians for teaching and research; by government
+research institutions such as Los Alamos National Lab in the US or
+CINECA in Italy; and by many commercial firms who use VTK to build or
+extend products.
+
+The origin of VTK is with the textbook "The Visualization Toolkit, an
+Object-Oriented Approach to 3D Graphics" originally published by
+Prentice Hall and now published by Kitware, Inc. (Third Edition ISBN
+1-930934-07-6). VTK has grown (since its initial release in 1994) to a
+world-wide user base in the commercial, academic, and research
+communities.
+
+Learning Resources
+==================
+
+* General information is available at the [VTK Homepage][].
+
+* Community discussion takes place on the [VTK Mailing Lists][].
+
+* Commercial [support][Kitware Support] and [training][Kitware Training]
+  are available from [Kitware][].
+
+* Doxygen-generated nightly reference documentation is
+  available [online][Doxygen].
+
+[VTK Homepage]: http://www.vtk.org
+[Doxygen]: http://www.vtk.org/doc/nightly/html
+[VTK Mailing Lists]: http://www.vtk.org/VTK/help/mailing.html
+[Kitware]: http://www.kitware.com/
+[Kitware Support]: http://www.kitware.com/products/support.html
+[Kitware Training]: http://www.kitware.com/products/protraining.php
+
+Reporting Bugs
+==============
+
+If you have found a bug:
+
+1. If you have a patch, please read the [CONTRIBUTING.md][] document.
+
+2. Otherwise, please join the one of the [VTK Mailing Lists][] and ask
+   about the expected and observed behaviors to determine if it is
+   really a bug.
+
+3. Finally, if the issue is not resolved by the above steps, open
+   an entry in the [VTK Issue Tracker][].
+
+[VTK Issue Tracker]: http://www.vtk.org/Bug
+
+License
+=======
+
+VTK is distributed under the OSI-approved BSD 3-clause License.
+See [Copyright.txt][] for details.
+
+[Copyright.txt]: Copyright.txt
+
+Contributing
+============
+
+See [CONTRIBUTING.md][] for instructions to contribute.
+
+[CONTRIBUTING.md]: CONTRIBUTING.md