Write a developer convenient security policy

SECURITY.md

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+# Security Policy
+
+## Downloading Mermaid from Trusted CDN
+
+The Django Mermaid downloads a specified version of Mermaid from a trusted https://cdnjs.cloudflare.com/ Content
+Delivery Network (CDN) repository.
+
+## Reporting Security Issues
+
+If you discover a security issue in the Django Mermaid project, please do not hesitate to report it by emailing us
+at [[email protected]](mailto:[email protected]). We take all security issues seriously and will respond to your
+report as soon as possible.
+
+Please provide as much information as possible when reporting a security issue, including steps to reproduce the issue,
+the expected behavior, and the actual behavior. This will help us to identify and fix the issue quickly.