Create SECURITY.md

This closes #1418

.github/SECURITY.md

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+# Security Policy
+
+
+## Reporting a Vulnerability
+
+The Gitblit team takes security bugs seriously. We appreciate your efforts to responsibly disclose your findings, and will make every effort to acknowledge your contributions.
+
+To report a security issue, please send an email to the following email address and include the word "SECURITY" in the subject line.
+
+```
+gitblitorg@gmail.com
+```