Update SECURITY.md

SECURITY.md

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 # Security Policy
 
+## Supported Versions
+
+Only new releases of this project will contain security updates. All clients should use the latest version of this project in their dependencies. There is no process in place to backport security fixes to previous releases. If you require a backport, please [create an issue](../../issues/new/choose) containing an explanation of why the latest version cannot be used.
+
 ## Reporting a Vulnerability
 
 Please report all potential security vulnerabilities using the [Report a vulnerability](../../security/advisories/new) button in the [Security](../../security) section of this repository.