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Update animation/README.md - part 3

Adds documentation for the Animation interface.

Change-Id: I64934a9adc4d2f63fac65bbb3977e7f5f10b8375
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2300259Reviewed-by: Xida Chen <xidachen@chromium.org>
Commit-Queue: Kevin Ellis <kevers@chromium.org>
Cr-Commit-Position: refs/heads/master@{#789462}