Add community file for GitHub security advisory feature.

Author: topeterk

.github/SECURITY.md

@@ -0,0 +1,17 @@
+# Security Policy
+
+
+## Supported Versions
+
+Versions that are currently being supported with security updates.
+
+| Version      | Supported          | Stale branch | 
+| ------------ | ------------------ | ------------ | 
+| 2.x          | :white_check_mark: |              | 
+| 1.x          | :white_check_mark: |              | 
+| (no release) | :x:                | xamarin      | 
+| < 1.14.0.0   | :x:                | visualbasic  | 
+
+## Reporting a Vulnerability
+
+Just report anything using regular GitHub issues.