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Add a README doc to educate people on developing new UI features that
support dark theme.

Bug: 941819
Change-Id: Ic364ab8dd4751a3e468b5d365b111fcbab682211
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1666611
Commit-Queue: Becky Zhou <huayinz@chromium.org>
Reviewed-by: Theresa <twellington@chromium.org>
Cr-Commit-Position: refs/heads/master@{#672267}