add security policy

Signed-off-by: Daijiro Fukuda <fukuda@clear-code.com>

Author: daipom

SECURITY.md

@@ -0,0 +1,16 @@
+# Security Policy
+
+## Supported Versions
+
+|       Version       |     Supported      |
+|---------------------|--------------------|
+| 6.0.x               | :white_check_mark: |
+| 5.2.x               | :x:                |
+| 5.1.x               | :x:                |
+| 5.0.x               | :white_check_mark: |
+| <= 4.x.x (td-agent) | :x:                |
+
+## Reporting a Vulnerability
+
+Please post your vulnerability report from the following page:
+https://github.com/fluent/fluent-package-builder/security/advisories