Add Readme.txt with legal info.

Readme.txt

+This repository contains the SwiftShader source code. It was licensed from
+TransGaming Inc. for use in conjunction with Google products, and should be
+treated as internal closed source.
+
+SwiftShader is a pure software implementation of OpenGL and Direct3D.
+It is used by Chrome for WebGL support with a blacklisted GPU, and by Looking
+Glass for running Windows applications on a VM without GPU support for
+streaming to Chromebooks.
+
+Watermark free versions of the SwiftShader builds should not be deployed on
+consumer systems! The watermark allows people to evaluate SwiftShader and
+license it from TransGaming if it suits their needs. Chrome has SwiftShader
+DLLs which are installed on consumer systems, but Chrome issues a license key
+to disable the watermark, so that with any other application the logo pops up.
+
+                                                              - capn@google.com
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