tutorial1-window on OS X

[guidorice]

If you're on MacOS, you can specify Vulkan (MoltenVK) as your desired backend instead of Metal by removing the wgpu = "0.5.0" and adding the following.

[dependencies.wgpu]
version = "0.5.0"
features = ["vulkan"]

I would also add something about what is the benefit or reason for adding features = ["vulkan"] .
My understanding is that without specifying vulkan, the pipeline on OS X is like:

wgpu-rs -> wgpu-core -> metal

adding features = ["vulkan"] then the pipeline on OS X is like this:

wgpu-rs -> wgpu-core -> vulkan -> moltenvk -> metal

But what would the tradoffs or pros/cons be of doing that? Confusing issues, there is actually an OS X SDK listed at https://vulkan.lunarg.com/sdk/home . I have a feeling there might be a lot of outdated information about Vulkan and OS X, on the web, but I am just learning. Let me know what you think about that, and I would be happy to submit a PR for this.

PS, also thanks so much for this awesome tutorial. Enjoying digging into it.

[guidorice]

@sotrh thanks much- I saw the Q&A over in gfx-rs/wgpu-rs#421
I can create a PR for learn-wgpu using kvark's info.