Add a security policy document (#1119)

The document outlines our patching and security support, along
with ways to report vulnerabilities.

SECURITY.md

@@ -0,0 +1,20 @@
+# Security Policy
+
+## Supported Versions
+
+Oban applies bug fixes only to the latest minor version. Security patches are
+available for the last 3 minor versions:
+
+| Oban Wersion | Support                        |
+| ------------ | ------------------------------ |
+| 2.17         | Bug fixes and security patches |
+| 2.16         | Security patches only          |
+| 2.15         | Security patches only          |
+
+## Announcements
+
+[Security advisories will be published on GitHub](https://github.com/sorentwo/oban/security).
+
+## Reporting Vulnerabilities
+
+[Please disclose security vulnerabilities via GitHub](https://github.com/sorentwo/oban/security).