Add security policy (#1342)

* Add security policy

Signed-off-by: Pedro Kaj Kjellerup Nacht <pnacht@google.com>

* Remove email, ask for version, description optional

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Signed-off-by: Pedro Kaj Kjellerup Nacht <pnacht@google.com>
diff --git a/SECURITY.md b/SECURITY.md
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+# Security Policy
+
+If you have discovered a security vulnerability in this project, please report it
+privately. **Do not disclose it as a public issue.** This gives us time to work with you
+to fix the issue before public exposure, reducing the chance that the exploit will be
+used before a patch is released.
+
+Please submit the report through [here](https://github.com/AOMediaCodec/libavif/security/advisories/new).
+
+Please provide the following information in your report:
+
+- Which version you're using
+- How to reproduce the issue
+- A description of the vulnerability and its impact (optional but appreciated)
+
+We ask that you give us 90 days to work on a fix before public exposure.