Add SECURITY.md

Author: tarcieri

SECURITY.md

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+# Security Policy
+
+## Supported Versions
+
+Security updates are applied only to the most recent release.
+
+## Reporting a Vulnerability
+
+If you have discovered a security vulnerability in this project, please report
+it privately. **Do not disclose it as a public issue.** This gives us time to
+work with you to fix the issue before public exposure, reducing the chance that
+the exploit will be used before a patch is released.
+
+Please disclose it at [security advisory](https://github.com/RustCrypto/stream-ciphers/security/advisories/new).
+
+This project is maintained by a team of volunteers on a reasonable-effort basis.
+As such, please give us at least 90 days to work on a fix before public exposure.