Add README.chromium

[kainino0x]

Metadata file for Chromium. Normally this would be stored in Chromium, but for historical reasons it's much easier to just put it here. Apologies :)
https://chromium.googlesource.com/chromium/src/+/HEAD/docs/adding_to_third_party.md#readme_chromium

Hopefully this works.

Issue: https://crbug.com/493761246

---

Requirements for PR author:

• [n/a] All missing test coverage is tracked with "TODO" or .unimplemented().
• [n/a] New helpers are /** documented */ and new helper files are found in helper_index.txt.
• [n/a] Test behaves as expected in a WebGPU implementation. (If not passing, explain above.)
• [n/a] Test have be tested with compatibility mode validation enabled and behave as expected. (If not passing, explain above.)

Requirements for reviewer sign-off:

• [n/a] Tests are properly located.
• [n/a] Test descriptions are accurate and complete.
• [n/a] Tests provide complete coverage (including validation control cases). Missing coverage MUST be covered by TODOs.
• [n/a] Tests avoid over-parameterization (see case count report).

When landing this PR, be sure to make any necessary issue status updates.

[github-actions]

Results for build job (at 3c7e67a):

 Test case/subcase counts did not change.

[Kangz]

LGTM but please see the comment on the relevant issue. What would moving into an src/ subdir entail?

[kainino0x]

whoops, shouldn't have autosubmitted