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This patch updates the readme to align with recent conversations
about what platform/, core/ and modules/ are and what they should be.

Change-Id: I2f4f0b50711cc2cd3cd455ad78c2fc5c96f53e1e
Reviewed-on: https://chromium-review.googlesource.com/1054190
Commit-Queue: Dmitry Gozman <dgozman@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: Jeremy Roman <jbroman@chromium.org>
Reviewed-by: Adithya Srinivasan <adithyas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#558099}