Create SECURITY.md

SECURITY.md

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+# Security Policy
+
+## Supported Versions
+
+We will only maintain one branch which is the master branch. Unless otherwise requested, no security patches will be applied to older Major/Minor versions. 
+
+## Reporting a Vulnerability
+
+For a vulnerability of low to no severity, which causing no threats to security, you may report it openly to us by [opening an issue](https://github.com/refraction-networking/utls/issues/new)
+
+If the vulnerability you are reporting inflicts some security impact, please [do so privately](https://github.com/refraction-networking/utls/security/advisories/new).