Add security policy

Author: joneugster

SECURITY.md

@@ -0,0 +1,17 @@
+# Security Policy
+
+## Supported Versions
+
+Currently this project does not support stable versions/releases,
+meaning security updates will be added to `main` and
+you are advised to keep up-to-date with the latest version on `main`.
+
+## Reporting a Vulnerability
+
+Please report any issues here on github, we aim to respond as soon as possible.
+The [npm package `lean4monaco`](https://github.com/hhu-adam/lean4monaco) is maintained by the same team,
+therefore you do not need to distinguish between issues concerning `lean4web` and `lean4monaco`.
+
+Alternatively, feel free to contact [Jon Eugster on the Lean Zulip](https://leanprover.zulipchat.com/#narrow/dm/385895-Jon-Eugster).
+
+The playground at [live.lean-lang.org](https://live.lean-lang.org/?from=lean) is maintained by the Lean FRO: contact@lean-fro.org