[F20] container readme

BUG=None
TEST=None

Change-Id: I302ba845cec200fcb2f1a8a9f01845dfdc734394
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/dev-util/+/3042339
Reviewed-by: Seewai Fu <seewaifu@google.com>
Reviewed-by: C Shapiro <shapiroc@chromium.org>
Commit-Queue: Derek Beckett <dbeckett@chromium.org>
Tested-by: Derek Beckett <dbeckett@chromium.org>
Auto-Submit: Derek Beckett <dbeckett@chromium.org>
1 file changed