Create SECURITY.md

Author: josevalim

SECURITY.md

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+# Security Policy
+
+## Supported Versions
+
+Currently we support security patches for the two most recent minor
+versions. For example, if Livebook v0.12.0 is released, we will
+backport security fixes for v0.12.x and v0.11.x. Once Livebook reaches
+v1.0, we plan to introduce more long-term security guarantees.
+If you have any questions, please reach our to them on Discussion page.
+
+## Reporting a Vulnerability
+
+To report a security vulnerability, you can send an email to `security@livebook.dev`
+or [privately disclose it on our GitHub page](https://github.com/livebook-dev/livebook/security).
+If you don't receive an acknowledgement of the report within 24h,
+feel free to ping us in our public forums (but without disclosing
+the contents of the vulnerability).