Add security policy in SECURITY.md

Author: asgrim

SECURITY.md

@@ -0,0 +1,25 @@
+# Security Policy
+
+## Supported Versions
+
+The following versions will receive security updates.
+
+| Version | Security updates   |
+| ------- | ------------------ |
+| 1.5.x   | :white_check_mark: |
+| 1.4.x   | :white_check_mark: |
+| 1.3.x   | :white_check_mark: |
+| < 1.3   | :x:                |
+
+## Reporting a Vulnerability
+
+Please do not publicly disclose security vulnerabilities.
+
+If you discover something that you think may be a vulnerability, please
+[report it **privately** on GitHub](https://docs.github.com/en/code-security/how-tos/report-and-fix-vulnerabilities/privately-reporting-a-security-vulnerability).
+
+ * Go to the [Security and Quality](https://github.com/php/pie/security) tab in the PIE repository.
+ * Click **Report a vulnerability** and fill in the form with as much information as possible.
+ * Hit submit, and we'll look into it as soon as possible.
+
+Thank you for responsibly disclosing issues in PIE 🥧