Write a README.md documenting the WPT canvas test generator

Fixed: 393561071
Change-Id: I4203084102cef2b4ec64991082ea6f71a932dd36
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6219875
Reviewed-by: Andres Ricardo Perez <[email protected]>
Commit-Queue: Jean-Philippe Gravel <[email protected]>
Cr-Commit-Position: refs/heads/main@{#1426392}

Author: graveljp