This CL adds the start of a document listing how things translate into
various backends.
Change-Id: Iebeb4488450b4f299e83d0de7ee124b441795e98
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/31240
Reviewed-by: David Neto <dneto@google.com>
Commit-Queue: dan sinclair <dsinclair@chromium.org>