Add design doc describing how to translate from SPIR-V Vulkan
style inputs/outputs to WGSL-style inputs as shader entry points,
and outputs as shader return value.
Change-Id: If0dc5f07542ccf2db95ed648a34f73a06e20a5a4
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/38961
Reviewed-by: Alan Baker <alanbaker@google.com>
Commit-Queue: David Neto <dneto@google.com>