Some people may happen across the security site expecting it to
contain documentation about cryptographic signatures for release
artifacts. Add a very brief paragraph to direct them to where these
are actually documented on the releases site.
Change-Id: Iaa3771ee21410e04f56d9d539d3f91423878514a
Signed-off-by: Jeremy Stanley <fungi@yuggoth.org>
Closes-Bug: #2119982