Create SECURITY.md (#1103)

SECURITY.md

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+# Security Policy
+
+## Supported Versions
+
+This is the list of versions of PurgeCSS which are currently being supported with security updates.
+
+| Version | Supported          |
+| ------- | ------------------ |
+| 5.0.x   | :white_check_mark: |
+| < 5.0   | :x:                |
+
+## Reporting a Vulnerability
+
+To report a vulnerability, please report it directly on GitHub. If you are not able to report it on GitHub,
+you can send an email with the details to [email protected]. The vulnerability report must include a
+proof-of-concept of the exploit, or at least a few pointers that can help us assess the risk level.
+Your report will be acknowledged within 2 business days.