Create SECURITY.md

Adds missing SECURITY.md, both for good practice and to fix a broken link in CONTRIBUTING.md

Author: UncleGrumpy

SECURITY.md

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+# Security Policy
+
+## Supported Versions
+
+The chart below depicts the versions of `atomvm_packbeam` which can be
+expected to receive security updates.
+
+| Version | Supported          |
+| ------- | ------------------ |
+| 0.7.x   | :white_check_mark: |
+| 0.6.x   | :x:                |
+| < 0.6   | :x:                |
+
+## Reporting a Vulnerability
+Please, do not use GitHub issues for vulnerability reports. Should you discover
+a security vulnerability in `atomvm_packbeam` please email the maintainer and/or
+project lead directly:
+
+Maintainer: Winford (UncleGrumpy), winford at object.stream
+Project Lead: Davide Bettio, davide at uninstall.it
+
+Your submission will be promptly reviewed and validated by a member of our team.