Salsa: Add a note about Treatment numbering

This just adds a little note about how treatments are numbered to try to
prevent any confusion for participants

BUG=chromium-os:186589
TEST=manually run with a dev server

Change-Id: I803aedbfc2aac84872b3ea503d9aa9c6e16f7c44
Signed-off-by: Charlie Mooney <charliemooney@chromium.org>