README for ash/public/cpp/external_arc

Bug: 1090616
Change-Id: I4d42843663c63bc57d7c2c5d6c92c1232c5f9455
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2250474Reviewed-by: James Cook <jamescook@chromium.org>
Commit-Queue: Mitsuru Oshima <oshima@chromium.org>
Cr-Commit-Position: refs/heads/master@{#782138}

ash/public/cpp/external_arc/README

+This directory contains the ash code that are accessible from the
+outside of ash (hence public), but also depends on components/arc
+and/or components/exo.