Add README file describing glcpp.

Mostly this is a place for me to write down the URLs of the GLSL and
C99 specifications that I need to write this code.

diff --git a/README b/README
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+glcpp -- GLSL "C" preprocessor
+
+This is a simple preprocessor designed to provide the preprocessing
+needs of the GLSL language. The requirements for this preprocessor are
+specified in the GLSL 1.30 specification availble from:
+
+http://www.opengl.org/registry/doc/GLSLangSpec.Full.1.30.08.pdf
+
+This specification is not precise on some semantics, (for example,
+#define and #if), defining these merely "as is standard for C++
+preprocessors". To fill in these details, I've been using the C99
+standard (for which I had a convenient copy) as available from:
+
+http://www.open-std.org/jtc1/sc22/wg14/www/docs/n1124.pdf