doc(security): add a security policy

Author: SyntaxRules

SECURITY.md

@@ -0,0 +1,16 @@
+# Security Policy
+
+## Supported Versions
+
+Security fixes are addressed for the following versions of Showdown.
+
+| Version | Supported          |
+| ------- | ------------------ |
+| 2.0.x   | :white_check_mark: |
+| 1.x.x   | :x: (Known security issue with yargs dependecy) |
+
+Showdown targets the node.js versions targeted in the [node.js release schedule](https://nodejs.org/en/about/releases/). Our test suite follows this release schedule. Consequently, older versions of node may become unusable.
+
+## Reporting a Vulnerability
+
+To report a vulnerability, please add an issue to our main github page: https://github.com/showdownjs/showdown/issues