Create SECURITY.md (#1142)

Author: ropwareJB

SECURITY.md

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+# Security Policy
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+## Reporting a Vulnerability
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+Report the vulnerability in the security [advisory section](https://github.com/deepstreamIO/deepstream.io/security) of the repository.
+
+If your submission is valid, a GitHub advisory will be published and if present in NPM or other released packages, a CVE will be requested.