Add security policy

.github/SECURITY.md

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+# Security Policy
+
+## Supported Versions
+
+We only support the latest production version, deployed to <https://content.minetest.net>.
+See the [releases page](https://github.com/minetest/contentdb/releases).
+
+## Reporting a Vulnerability
+
+We ask that you report vulnerabilities privately, by contacting rubenwardy,
+to give us time to fix them. You can do that by using one of the methods outlined in the following link:
+
+* https://rubenwardy.com/contact/
+
+Depending on severity, we will either create a private issue for the vulnerability
+and release a security update, or give you permission to file the issue publicly.
+
+For more information on the justification of this policy, see
+[Responsible Disclosure](https://en.wikipedia.org/wiki/Responsible_disclosure).