Create SECURITY.md (#1106)

* Create SECURITY.md

Fixes https://github.com/hyperium/tonic/issues/1100

* Update SECURITY.md

SECURITY.md

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+# Security Policy
+
+tonic (and related projects in hyperium) uses the same security policy as the [Tokio project][tokio-security].
+
+## Report a security issue
+
+The process for reporting an issue is the same as the [Tokio project][tokio-security]. This includes private reporting via security@tokio.rs.
+
+[tokio-security]: https://github.com/tokio-rs/tokio/security/policy