CookieStore: Add design doc to README

Change-Id: I58fc3bb0ea61cb2a04bde0b3c54e8ff8bd1ab00c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2414678
Commit-Queue: Ayu Ishii <ayui@chromium.org>
Reviewed-by: Victor Costan <pwnall@chromium.org>
Cr-Commit-Position: refs/heads/master@{#808410}

third_party/blink/renderer/modules/cookie_store/README.md

-This module implements the
-[Cookie Store API](https://github.com/WICG/cookie-store). The
-implementation is under active development, and its progress is tracked in
-[this OWP Launch bug](https://crbug.com/729800)
+# Cookie Store API
+
+This module contains the implementation of the
+[Cookie Store API](https://wicg.github.io/cookie-store).
+
+## Testing
+
+The Cookie Store API is primarily tested in
+[Web Platform Tests](https://source.chromium.org/chromium/chromium/src/+/master:third_party/blink/web_tests/external/wpt/cookie-store/).
+
+## Design Documents
+
+Please refer to the
+[explainer](https://github.com/WICG/cookie-store/blob/main/explainer.md) and
+[original design documentation](https://docs.google.com/document/d/1ak6JzOMMO5q3dXvu4mHFWR-LLvaDc09XDvdeJZLtZd4/)
+for more details.