Revise SECURITY.md for supported versions and reporting

Updated the security policy to reflect supported versions and reporting process.

Author: Augus1999

.github/SECURITY.md

@@ -0,0 +1,13 @@
+# Security Policy
+
+## Supported Versions
+
+| Version | Supported          |
+| ------- | ------------------ |
+| 3.x.x   | :white_check_mark: |
+| 2.x.x   | :x:                |
+| 1.x.x   | :x:                |
+
+## Reporting a Vulnerability
+
+Please report in [Issues](https://github.com/Augus1999/pyPDFeditor-GUI/issues) (I suppose no issue would be fatal so it's okey to make it public).