Add security guidelines

SECURITY.md

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+
+# Security Policy
+
+The NEST simulator is research oriented software and as such is expected to
+run mainly in well protected environments. In case it is found that this
+software can be used to violate security mechanisms, developers will try to
+provide patches that mitigate the risk of this misuse.
+
+The current policy is explained in SECURITY.md in the nest-simulator master branch.
+The policy can also be found at <https://github.com/nest/nest-simulator/security/policy>.
+
+## Supported Versions
+
+Generally only the latest version will receive security related updates. In severe
+cases developers may create a backport of the corresponding patch for the
+previous version, if it is also affected.  Currently the following versions are
+being supported with security updates:
+
+| Version  | Supported          |
+| -------- | ------------------ |
+| <=2.14.0 | :x:                |
+| 2.16.x   | :white_check_mark: |
+| 2.18.x   | :heavy_check_mark: |
+
+## Reporting a Vulnerability
+
+For reporting a vulnerability please create a security advisory on the
+nest/nest-simulator [Security
+Advisories](https://github.com/nest/nest-simulator/security/advisories) page.
+You need a GitHub account to create an advisory.
+
+Developers will then contact the reporter in a timely manner to assess severity
+and further handling via [Security
+Advisories](https://github.com/nest/nest-simulator/security/advisories) or as
+normal [Issue](https://github.com/nest/nest-simulator/issues) in non-critical
+cases.
+