Enable private reporting of vulnerabilities or document how to report vulnerabilities

[maxammann]

I wanted to report a security issue, but failed to find a contact. @holgerd77 did not respond to an email I sent. I am aware that this repository is no longer maintained, but first wanted to confirm that posting the issue as GitHub issue is fine.

There are at least two possibilities:

• Enable GitHub private reporting
• Create a security policy (SECURITY.md) which documents how and who to contact.

If there is no response to this issue until 21. of August 2023, a GitHub issue about the vulnerability will be created.