Add security policy (#127)

SECURITY.md

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+# Security Policy
+
+If you think you have identified a security issue within this project, please **do not open a public issue**.
+
+To responsibly report a security issue,
+
+1. Navigate to the main page of the repository on GitHub.
+2. Under the repository name, click  Security. If you cannot see the "Security" tab, select the  dropdown menu, and then click Security.
+3. Click [Report a vulnerability](https://github.com/SRv6d/rpsl-rs/security/advisories/new) to open the advisory form.
+4. Fill in the advisory details form and click Submit report.
+
+Thank you.