Add DO-NOT-USE.txt notice to the repo

Add DO-NOT-USE.txt to notify users not to submit new firmware to this
project.

BUG=none
TEST=none

Change-Id: Ib795ad5e02257ae96decd2b956765596791c82b9
Reviewed-on: https://chromium-review.googlesource.com/368100
Commit-Ready: Brian Norris <briannorris@chromium.org>
Tested-by: Brian Norris <briannorris@chromium.org>
Reviewed-by: Brian Norris <briannorris@chromium.org>
1 file changed