Create SECURITY.md (PostHog#6009)

* Create SECURITY.md

* Update SECURITY.md

Co-authored-by: Michael Matloka <dev@twixes.com>

Co-authored-by: Michael Matloka <dev@twixes.com>

Author: timgl

SECURITY.md

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+# Security Policy
+
+## Supported Versions
+
+We always recommend using the latest version of PostHog to ensure you get all security updates.
+
+## Reporting a Vulnerability
+
+Please report security vulnerabilities to hey@posthog.com.
+
+We currently do not operate a bug bounty program, but we will generously reward you with merch for any actionable security vulnerabilities found.