Add SECURITY.md (#5)

Author: RomainCscn

SECURITY.md

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+# Security
+
+## Reporting a Vulnerability
+
+If you discover a security vulnerability, please email **security@linear.app**. See our [vulnerability reporting guidelines](https://linear.app/security/vulnerability) for details on scope and expectations.
+
+Please **do not** use public GitHub issues to report security vulnerabilities.
+
+## Security Practices
+
+For information about Linear's security certifications, compliance, and practices, visit [linear.app/security](https://linear.app/security).