Create SECURITY.md

Author: mrichar1

SECURITY.md

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+# Security Policy
+
+## Supported Versions
+
+
+| Version | Supported          |
+| ------- | ------------------ |
+| 5.x   | :white_check_mark: |
+| 4.5.x   | :white_check_mark:|
+
+Version 4.5.x releases still support Vue2. 5x supports only Vue3.
+Security updates will continue to be made as 4.5.x releases until the end of 2021, but no feature updates will be made.
+
+## Reporting a Vulnerability
+
+If you suspect a vulnerability in `jsonapi-vuex` please report it by emailing the lead developer directly: m.richardson@ed.ac.uk