docs: add a security policy (#267)

Author: ForbesLindesay

SECURITY.md

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+
+
+# Security Policy
+
+## Supported Versions
+
+Only the latest version of each package is supported. Security patches will not be released for old major versions.
+
+## Reporting a Vulnerability
+
+To report a security vulnerability, please use the
+[Tidelift security contact](https://tidelift.com/security).
+Tidelift will coordinate the fix and disclosure.