Update SECURITY.md

SECURITY.md

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 # Security Policy
 
+## Supported Versions
+
+Only the latest released version is supported
+(see https://github.com/eliben/pycparser/tags). Updates will be made in
+the `master` branch.
+
+## Reporting a Vulnerability
+
 To report a security issue, please disclose it at [security advisory](https://github.com/eliben/pycparser/security/advisories/new).
 
 We will respond within 14 working days of your submission. If the issue is confirmed as a vulnerability, we will open a Security Advisory and acknowledge your contributions as part of it. This project follows a 90 day disclosure timeline.