add SECURITY.md file.

SECURITY.md

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+# Security Policy
+
+## Reporting a Vulnerability
+
+If there are any vulnerabilities in **m1k1o/neko**, don't hesitate to _report them_.
+
+1. Send an email to `security@m1k1o.net`.
+
+2. Describe the vulnerability.
+
+   If you have a fix, that is most welcome -- please attach or summarize it in your message!
+
+3. We will evaluate the vulnerability and, if necessary, release a fix or mitigating steps to address it. We will contact you to let you know the outcome, and will credit you in the report.
+
+   Please **do not disclose the vulnerability publicly** until a fix is released!
+
+4. Once we have either a) published a fix, or b) declined to address the vulnerability for whatever reason, you are free to publicly disclose it.
+
+We appreciate your help in keeping Neko secure.