samples: Add ${DATETIME} to the message aswell

Without ${DATETIME}, we can too easily run into a duplicate status
exception. We don't want that for these kinds of 'alerts'.

Signed-off-by: Gergely Nagy <algernon@balabit.hu>
README: Correct the blog post URL

Signed-off-by: Gergely Nagy <algernon@balabit.hu>