docs: add security policy

SECURITY.md

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+# Security Policy
+
+## Supported Versions
+
+Before v1.0 only the latest version of Excalibur.js is supported.
+
+## Reporting Security Issues
+
+The Excalibur.js team and its community take security bugs seriously. We appreciate your efforts to responsibly disclose your findings, and will make every effort to acknowledge your contributions.
+
+To report a security issue, please use the GitHub Security Advisory ["Report a Vulnerability"][create-new-report] button.
+
+The Excalibur.js team will send a response indicating the next steps in handling your report. After the initial reply to your report, the security team will keep you informed of the progress towards a fix and full announcement, and may ask for additional information or guidance.
+
+
+[create-new-report]: https://github.com/excaliburjs/Excalibur/security/advisories/new