Create SECURITY.md

Author: pjfanning

SECURITY.md

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+# Security Policy
+
+## Supported Versions
+
+By default, only the latest version of the library is supported. Security patches may be backported to older versions, but only in exceptional circumstances.
+
+## Reporting a Vulnerability
+
+* Suspected security vulnerabilities should not be discussed publicly. Do not open an issue using the normal issue tracker.
+* Create your own fork of this project
+* Create a [Security Advisory](https://docs.github.com/en/code-security/security-advisories/creating-a-security-advisory) in your fork. Do not worry about getting all the settings correct initially.
+* [Grant access](https://docs.github.com/en/code-security/security-advisories/adding-a-collaborator-to-a-security-advisory) to the my username (pjfanning) so that I can see the description of the issue and comment.
+* If I accept that there is a vulnerability, I move the Security Advisory to this project and add all the parties from the fork advisory as collaborators.
+* I will try to get a fix, a release and CVE assignment done as quickly as I can.