Update documentation for descriptor set support in MSL/HLSL.
Update documentation for descriptor set support in MSL/HLSL.

Ancient outdated README.

KhronosGroup/SPIRV-Cross,KhronosGroup/SPIRV-Cross,KhronosGroup/SPIRV-Cross,KhronosGroup/SPIRV-Cross,KhronosGroup/SPIRV-Cross,KhronosGroup/SPIRV-Cross