Add SECURITY.md document. Closes #1790

doc/SECURITY.md

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+# Security Policy
+
+Owncast appreciates efforts to improve the security of the software
+and follow the [GitHub coordinated disclosure of security vulnerabilities](https://docs.github.com/en/code-security/security-advisories/about-coordinated-disclosure-of-security-vulnerabilities#about-reporting-and-disclosing-vulnerabilities-in-projects-on-github)
+for responsible disclosure and prompt mitigation.
+
+## Supported Versions
+
+The latest version of Owncast is seen as the supported version. As a small project we are unable to support previous versions and urge users of the software to stay up to date.
+
+## Reporting a Vulnerability
+
+To report a security issue with Owncast, [open an issue](https://github.com/owncast/owncast/issues/new
+) on the Owncast GitHub repository and *do not* mention vulnerability details in the issue. If you have a preferred next step on where to discuss the details of the disclosure, please mention that in the issue if it's appropriate for those details to be public.
+
+You may optionally [email Gabe](mailto:gabek@real-ity.com) to alert him directly and provide specifics on how you wish to disclose the details of the issue.
+
+Owncast may open a draft [GitHub Security Advisory](https://docs.github.com/en/code-security/security-advisories/creating-a-security-advisory)
+  to discuss the vulnerability details in private if it is warranted.