Create SECURITY.md

SECURITY.md

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+# Security Policy
+
+## Supported Versions
+
+We provide security updates for the following releases.
+For more on Spack's release structure, see
+[`README.md`](https://github.com/spack/spack#releases).
+
+
+| Version | Supported          |
+| ------- | ------------------ |
+| develop | :white_check_mark: |
+| 0.16.x  | :white_check_mark: |
+
+## Reporting a Vulnerability
+
+To report a vulnerability or other security
+issue, email maintainers@spack.io.
+
+You can expect to hear back within two days.
+If your security issue is accepted, we will do
+our best to release a fix within a week. If
+fixing the issue will take longer than this,
+we will discuss timeline options with you.