[ScrollTimeline] Add README.md to fast/animation/scroll-animations

This CL adds a README.md to the fast/animations/scroll-animations
directory, explaining when that directory should be used vs
external/wpt/scroll-animations (aka, rarely).

Bug: 911254
Change-Id: Id24ee770dcbe070b1f89ffe925edbacbba4a673c

TBR=yigu@chromium.org

Change-Id: Id24ee770dcbe070b1f89ffe925edbacbba4a673c
Reviewed-on: https://chromium-review.googlesource.com/c/1367825
Reviewed-by: Stephen McGruer <smcgruer@chromium.org>
Reviewed-by: Yi Gu <yigu@chromium.org>
Commit-Queue: Stephen McGruer <smcgruer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#615535}
1 file changed