ICU-22410 Add security policy

Signed-off-by: Pedro Kaj Kjellerup Nacht <[email protected]>
Co-authored-by: Markus Scherer <[email protected]>

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 bug report as an [ICU Jira issue](https://icu.unicode.org/bugs).
+Be sure to set "Security level: Sensitive".
+This ensures the report will only be visible to the ICU team.
+
+Please provide the following information in your report:
+
+- A description of the vulnerability and its impact
+- How to reproduce the issue
+
+This project is maintained by volunteers on a reasonable-effort basis. As such, we ask
+that you give us 90 days to work on a fix before public exposure.
+
+
+> Copyright © 2023 and later Unicode, Inc. and others. All Rights Reserved.
+Unicode and the Unicode Logo are registered trademarks
+of Unicode, Inc. in the U.S. and other countries.
+[Terms of Use and License](http://www.unicode.org/copyright.html)