Create SECURITY.md (#1424)

* Create SECURITY.md

According to https://github.com/ossf/scorecard/blob/5758364c82f7fc72b256f9a8cfc89dc550d7dd66/docs/checks.md#security-policy

Ref #1298

* Update SECURITY.md

SECURITY.md

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+# Security Policy
+
+##  Sponsoring the project
+
+This project is maintained by a small team of two and therefore lacks the resource to provide security fixes in a timely manner.
+
+If you have important business(es) that relies on this project, please consider sponsoring the project so that the maintainer(s) can commit to providing such service.
+
+Please refer to https://github.com/sponsors/actions-runner-controller for available tiers.
+
+## Supported Versions
+
+| Version | Supported          |
+| ------- | ------------------ |
+| 0.23.0  | :white_check_mark: |
+| < 0.23.0| :x:                |
+
+## Reporting a Vulnerability
+
+To report a security issue, please email ykuoka+arcsecurity(at)gmail.com with a description of the issue, the steps you took to create the issue, affected versions, and, if known, mitigations for the issue.
+
+A maintainer will try to respond within 5 working days. If the issue is confirmed as a vulnerability, a Security Advisory will be opened. This project tries to follow a 90 day disclosure timeline.