Create SECURITY.md (#19)

SECURITY.md

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+# Security Policy
+
+The Trixi.jl development team takes security issues seriously. We appreciate
+all efforts to responsibly disclose any security issues and will make every
+effort to acknowledge contributions.
+
+
+## Supported Versions
+
+The current stable release following the interpretation of
+[semantic versioning (SemVer)](https://julialang.github.io/Pkg.jl/dev/compatibility/#Version-specifier-format-1)
+used in the Julia ecosystem is supported with security updates.
+
+
+## Reporting a Vulnerability
+
+To report a security issue, please use the GitHub Security Advisory
+["Report a Vulnerability"](https://github.com/trixi-framework/TrixiBase.jl/security/advisories/new)
+tab.
+
+We will send a response indicating the next steps in handling your report.
+After the initial reply to your report, we will keep you informed of the
+progress towards a fix and full announcement, and may ask for additional
+information or guidance.
+
+Please report security bugs in third-party modules directly to the person
+or team maintaining the module.
+
+Public notifications of vulnerabilities will be shared in community channels
+such as Slack.