Add initial security policy (#4379)

Signed-off-by: Josef Andersson <janderssonse@proton.me>
Co-authored-by: Junegunn Choi <junegunn.c@gmail.com>

SECURITY.md

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+# Security Reporting
+
+If you wish to report a security vulnerability privately, we appreciate your diligence. Please follow the guidelines below to submit your report.
+
+## Reporting
+
+To report a security vulnerability, please provide the following information:
+
+1. **PROJECT**
+   - Include the URL of the project repository - Example: <https://github.com/junegunn/fzf>
+
+2. **PUBLIC**
+   - Indicate whether this vulnerability has already been publicly discussed or disclosed.
+   - If so, provide relevant links.
+
+3. **DESCRIPTION**
+   - Provide a detailed description of the security vulnerability.
+   - Include as much information as possible to help us understand and address the issue.
+
+Send this information, along with any additional relevant details, to <junegunn.c AT gmail DOT com>.
+
+## Confidentiality
+
+We kindly ask you to keep the report confidential until a public announcement is made.
+
+## Notes
+
+- Vulnerabilities will be handled on a best-effort basis.
+- You may request an advance copy of the patched release, but we cannot guarantee early access before the public release.
+- You will be notified via email simultaneously with the public announcement.
+- We will respond within a few weeks to confirm whether your report has been accepted or rejected.
+
+Thank you for helping to improve the security of our project!