Create security policy

SECURITY.md

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+# Security Policy
+
+## Supported Versions
+
+I support only the last published version and the last development version (last commit).
+
+## Reporting a Vulnerability
+
+To report a vulnerability, create a private bug at https://sourceforge.net/p/adminer/bugs-and-features/new/?private=1.
+
+I handle security issues with top priority. If you don't hear from me in a week then please ping the bug. Once I accept the bug, the fix should be available and new version released within days. I will mark the bug as public after releasing a new version or declining the bug.