Add a SECURITY.md file

SECURITY.md

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+# Security Policy
+
+## Supported Versions
+
+| Version | Supported          |
+| ------- | ------------------ |
+| 2.2.+   | :white_check_mark: |
+| < 2.2   | :x:                |
+
+## Reporting a Vulnerability
+
+Hi! If you've found something off, we'd be more than happy if you would report it via security@speckle.systems. We will work together with you to correctly identify the cause and implement a fix. Thanks for helping make Speckle safer!