intel/genxml: Add README
intel/genxml: Add README

I've had people ask about the design of the pack functions, for example,
why aren't we using bitfields. I wrote up a bit of background on why and
how we ended up with the current design and we might as well keep that
with the code.

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler