Next SDK we will be changing branch/tag naming scheme from sdk-1.x.yyy to vulkan-sdk-1.x.yyy

This is intended to clarify that developers using Vulkan are likely to
need only the header files, not the scripts and other material in the
repository which are used by the SDK and other downstream ecosystem
components.

Closes #407

CONTRIBUTING.md, issue template, and PR template all cover this.

Vulkan::Registry behaves differently in `add_subdirectory` vs
`find_package` builds. Which can result in build failures.

Also Vulkan::Registry never made sense as a CMake target and
was very brittle in numerous ways.

Better to provide a `VULKAN_HEADERS_REGISTRY_DIRECTORY` to users
instead and remove Vulkan::Registry completely.

closes #351

Ensures the non-API headers compile correctly

The old description referred to a person who no longer is responsible for those files. 
I prefer to update the link to refer to the Vulkan-Loader so future changes in ownership
don't require updating this repo.

Several files originate from unknown (to @oddhack) sources and that
should be fixed before accepting this PR.

The BEGIN_RANGE, END_RANGE, RANGE_SIZE enums have now been removed from the headers, per https://github.com/KhronosGroup/Vulkan-Docs/issues/1230, so remove the "Advance Notice" from the README.