docs: add SECURITY.md (#2591)

We should make it explicit that tracing uses the security policy of Tokio.

Author: Darksonn

Diff for: SECURITY.md

@@ -0,0 +1,9 @@
+# Security Policy
+
+Tracing is part of the Tokio project and uses the same security policy as [Tokio][tokio-security].
+
+## Report a security issue
+
+The process for reporting an issue is the same as for [Tokio][tokio-security]. This includes private reporting via [email protected].
+
+[tokio-security]: https://github.com/tokio-rs/tokio/security/policy