added comment to describe Windows 8.1 and Visual Studio requirement for d3dcompile
added comment to describe Windows 8.1 and Visual Studio requirement for d3dcompile

MSOpenTech/angle-win8.0,MSOpenTech/angle-win8.0,stammen/angleproject,MSOpenTech/angle-win8.0,stammen/angleproject,MSOpenTech/angle-win8.0,stammen/angleproject,stammen/angleproject