Merge pull request #68 from guidorice/docs-beginner-vulkan-backend

Clarify what the wgpu-rs's vulkan feature does on Mac OS and other platforms

docs/beginner/tutorial1-window/README.md

@@ -14,7 +14,14 @@ wgpu = "0.5.0"
 futures = "0.3.4"
 ```
 
-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.
+### Vulkan Portability Layer
+
+You may also want to make the Vulkan backend available on platforms where it is by default not, e.g. Mac OS. The reason
+you might want to enable the Vulkan backend is if you are doing cross-platform development and you need the
+Vulkan validation layers output. 
+
+To enable the Vulkan backend, add this new section to `Cargo.toml`, and do not forget to remove `wgpu` from the earlier
+`[dependencies]` section.
 
 ``` toml
 [dependencies.wgpu]
@@ -22,6 +29,10 @@ version = "0.5.0"
 features = ["vulkan"]
 ```
 
+- This may not be necessary because normally `wgpu-rs` validation will catch problems. In fact that is one of it's
+  design goals.
+- This is not intended for shipping code. 
+- See also [gfx-portability](https://github.com/gfx-rs/portability).
 
 ## The code
 There's not much going on here yet, so I'm just going to post the code in full. Just paste this into you're `main.rs` or equivalent.