Create SECURITY.md [skip ci] (#487)

Author: nakamura-to

SECURITY.md

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+# Security Policy
+
+We take all security bugs very seriously.
+
+## Supported Versions
+
+The community will fix security bugs for the latest major.minor version.
+
+| Version    | Supported          |
+| ---------- | ------------------ |
+| latest 2.x | :white_check_mark: |
+| older 2.x  | :x:                |
+| latest 1.x | :white_check_mark: |
+| older 1.x  | :x:                |
+
+We may fix the vulnerability to older versions depending on the severity of the issue and the age of the release,
+but we are only committing to the latest version released.
+
+## Reporting a Vulnerability
+
+Email to the below address:
+
+- toshihiro.nakamura at gmail.com
+
+Or contact us with Twitter:
+
+- [@domaframework](https://twitter.com/domaframework)