Adding chapter on HLSL shader model coverage about vulkan-guide

[sfricke-samsung]

So I 100% agree there should be an HLSL chapter, I have zero knowledge about HLSL, so I never felt confident writing one up... would be grateful if you wanted to start a chapter, don't need much, but as you said, would be at least a place holder for future things if people want to add

[SaschaWillems]

I'm not too knowledgeable with HLSL too, but I could do a basic chapter on how HLSL fits into Vulkan and a table that maps shader model versions to Vulkan features. I'll try to whip up something and do a PR.