chore: add basic security notice

SECURITY.md

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+# Reporting Security Issues
+
+The Lakera team takes security issues in Lakera projects seriously.
+
+We appreciate your efforts to follow responsible disclosure projects and will make every effort to acknowledge your contributions.
+
+Please use the project's [GitHub Security tab](https://github.com/lakeraai/chainguard/security/) or email `security` at `lakera.ai` to report any vulnerabilities.
+
+If you have any questions or concerns, please email `security` at `lakera.ai`.
+
+After your initial report, the team will do its best to keep you updated as our team works towards a fix and an announcement. We may ask for more information or guidance from you.