Add core/editing/finder/README.md

It was written by Gemini, and edited by tkent@.
This CL has no behavior changes.

Change-Id: I51f4fb770a81ec86c3e48473a7d094e0cc8f6c88
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6849871
Auto-Submit: Kent Tamura <tkent@chromium.org>
Reviewed-by: Koji Ishii <kojii@chromium.org>
Commit-Queue: Koji Ishii <kojii@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1502555}