Add SECURITY.md, including the versions affected by the 1.4.0 nonce fix

SECURITY.md

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+# Security Policy
+
+netcode implements an encrypted, connection-oriented protocol over UDP. It parses
+untrusted data straight off the wire — packets, connect tokens, and the challenge
+exchange — and it holds the keys, so we take memory-safety and protocol bugs seriously.
+
+## Reporting a vulnerability
+
+**Please do not report security issues in public GitHub issues or pull requests.**
+
+Report privately through either channel:
+
+- **GitHub private vulnerability reporting** (preferred): on this repository, go to the
+  **Security** tab → **Report a vulnerability**. This opens a private advisory visible only
+  to the maintainers.
+- **Email**: glenn@mas-bandwidth.com.
+
+Please include enough detail to reproduce: the affected component and version/commit, a
+description of the flaw, and — where possible — a proof-of-concept input or a small patch.
+Fuzzing crash artifacts (a crashing input file plus the target name) are ideal.
+
+We will acknowledge your report, keep you updated on our assessment, and coordinate
+disclosure timing with you. We prefer coordinated disclosure and will credit reporters who
+wish to be named.
+
+## Scope
+
+In scope — bugs in this repository:
+
+- the netcode library itself (`netcode.c`, `netcode.h`);
+- the pruned **libsodium** subset under `sodium/` **as vendored** (e.g. an amalgamation or
+  pruning mistake). Vulnerabilities in upstream libsodium itself should be reported to the
+  [libsodium project](https://github.com/jedisct1/libsodium); we track upstream and pull in
+  fixes — see `sodium/NOTES.md` for the review log.
+
+Especially of interest: memory-safety issues (out-of-bounds read/write, use-after-free,
+overflow) reachable from a received packet or connect token; and protocol flaws that let a
+peer bypass authentication, encryption, or replay protection.
+
+The protocol itself is specified in `STANDARD.md`. A flaw in the *specification* — as
+opposed to this implementation of it — is in scope and is more valuable to us, because it
+affects every implementation of netcode rather than one.
+
+## Known issue: nonce reuse between global and per-client packets (fixed in 1.4.0)
+
+**Affected: netcode 1.3.5 and earlier. Fixed in 1.4.0.**
+
+Global packets (connection challenge, connection denied) encrypt with the same
+per-connect-token server→client key as per-client packets. The server's global packet
+sequence was seeded only when the server was *created*, not when it was *started*, so a
+server that was stopped and started again could emit global packets at sequence numbers
+already used under the same key. Since netcode uses the packet sequence as the AEAD nonce,
+that is nonce reuse.
+
+Fixed by re-seeding the global sequence on start as well as on create
+([`dc21b70`](https://github.com/mas-bandwidth/netcode/commit/dc21b70)), first released in
+**1.4.0**.
+
+### If you are using an affected version
+
+Upgrade to 1.4.0 or later. If you cannot, avoid restarting a server in-process; a fresh
+process is unaffected because the sequence is seeded at creation.
+
+### Where affected versions can still be obtained
+
+We are recording these because we cannot remove them all, and a user has no other way to
+find out:
+
+- **Conan (legacy `center.conan.io`)** — serves `yojimbo/1.2.1`, which vendors an affected
+  netcode. That remote is frozen: nobody, including us, can update or withdraw it.
+- **Debian mentors** — has served `netcode 1.3.5+ds-1`. Only the uploading account can
+  supersede it.
+
+**Ports:** `netcode.go` carries a `retract [v1.0.0, v1.0.2]` directive in its `go.mod`, so
+`go get` will warn. `netcode.rs` and the C library are fixed from 1.4.0 / 1.1.0 onward.
+
+**yojimbo** vendors netcode. A yojimbo release is affected if the netcode it carries is
+1.3.5 or earlier — that is **yojimbo 1.6.3 and earlier**; yojimbo 1.7.0 was the first to
+vendor netcode 1.4.0.
+
+## Supported versions
+
+Security fixes land on the latest release. We do not backport to older release lines.