feat: add project security document

src/SECURITY.md

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+# Security Policy
+
+## Supported Versions
+
+The application white check mark is still in support, without it means there
+version will no longer have a update including security fixture. That's beacuse
+currently I have a job and a hard learning plain, which means I don't have too
+much time to dealing with this project now, sincerely, I am sorry for that.
+Maybe this situation will get better in the future. So it is recommand to use
+the latest version.
+
+| Version | Supported          |
+| ------- | ------------------ |
+| 1.x.x   | :x:                |
+| 2.x.x   | :x:                |
+| 2.1.x   | :white_check_mark: |
+
+## Platform & Library Support
+
+According to security concern and my limiting time to contributing, GpgFrontend
+will gradually drop the support of some old platforms. 
+
+Starting from v1.0.0, x32 operating system is no longer support. Mainly because
+I have limit time and the fact that x32 machime is getting less and less in the
+pc market, so I plan to stop spending tons of time dealing with lots of complex
+problem which might made me crazy. x64 is just enough, or you can use gpg4usb.
+
+Starting from v2.0.1, Ubuntu 16.04 is no longer support. Mainly because this
+Ubuntu 16.04 LTS's life time had come into an end at during that time. Keep
+using this version of Ubuntu is not safty anymore.
+
+Starting from v2.1.0, macOS 10.15 is no longer support. Mainly because this
+system will no longer support by github action in the near future. Also, I think
+this version is old enough to drop it out, and Apple might not longer to support
+it soon.
+
+## Reporting a Vulnerability
+
+If you want to report a vulnerability, it's not good to raise an issue in
+public. You should send an email to eric@bktus.com to contract me. In the email,
+You can describe the vulnerability you have just discovered and request a
+hot-fix. Or you can just provide a git patch to fix it and send it to me using
+the email. This is a better way to solve the vulnerability as quick as possible.