Create SECURITY.md

Author: okuramasafumi

SECURITY.md

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+# Security Policy
+
+## Supported Versions
+
+| Version | Supported          |
+| ------- | ------------------ |
+| 1.x.y   | :white_check_mark: |
+| < 1.0   | :x:                |
+
+## Reporting a Vulnerability
+
+If you find a vulnerability of Alba, please contact me (OKURA Masafumi) via [email](masafumi.o1988@gmail.com). I'll report back within a few days.