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No change to logic, only docs.

This adds general documentation for how to launch Dev UI, and provides
information for how Dev UI interacts with Finch.

Fixed: 1058571
Test: tools/md_browser/md_browser.py
Test: Upload to gerrit > open file > click "gitiles"
Change-Id: Icc7234807935d764886a66305ff8cd5653701f5e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2123334Reviewed-by: Hazem Ashmawy <hazems@chromium.org>
Commit-Queue: Nate Fischer <ntfschr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#755053}