dominicrume/cantor8_kya_rails: docs/drafts/post-sig-security.md

Post-To: https://forum.canton.network/c/sigs/sig-security/79 (New Topic. Title: "Refusal evidence on the ledger: what a stolen single key does and does not leave behind") The 26 October advisory sent me to test something rather than to read it. It says: "Start monitoring the network for…