Add .readme back to recyclerview/src to make gclient happy

Gclient is not happy with an empty source folder.

BUG=NONE

Change-Id: I909bd753679e60be4aee327fa5c16a3369248cd8
Reviewed-on: https://chromium-review.googlesource.com/333708
Reviewed-by: Ian Wen <ianwen@chromium.org>
Tested-by: Ian Wen <ianwen@chromium.org>
Commit-Queue: Ian Wen <ianwen@chromium.org>
Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>