docs: add minimal SECURITY.md (#122)

Co-authored-by: Devin AI <158243242+devin-ai-integration[bot]@users.noreply.github.com>

Author: jamesbhobbs

SECURITY.md

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+# Security
+
+Report vulnerabilities to security@deepnote.com. If you need to encrypt your message, use our [PGP key](https://deepnote.com/.well-known/pgp-key.txt).
+
+For full security policy and contact information, see [security policy](https://deepnote.com/.well-known/security.txt)