Create SECURITY.md

SECURITY.md

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+# Security Policy
+
+## Supported Versions
+
+| Version | Supported          |
+| ------- | ------------------ |
+| <= 1.7  | :x:                |
+| 1.8.x   | :white_check_mark: |
+| 2.0.x   | :white_check_mark: |
+
+## Reporting a Vulnerability
+
+Please use email to contact the project's main contributors with any vulnerability you become aware of.
+
+    Yoshifumi Kawai <ils@neue.cc>; Andrew Arnott <andrewarnott@gmail.com>