cros-publish: add readme file

only added steps for gcs-publish now. steps for tko and rdb will be
added later.

BUG=b:241053779
TEST=None

Change-Id: I265ad50ce981c86c0e458a66fd5e5f172312be23
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/dev-util/+/3894479
Tested-by: Azizur Rahman <azrahman@google.com>
Auto-Submit: Azizur Rahman <azrahman@google.com>
Reviewed-by: Jaques Clapauch <jaquesc@google.com>
Commit-Queue: Jaques Clapauch <jaquesc@google.com>