Create SECURITY.md

[ranocha]

This shows that we're taking this open-source project seriously. It is considered to be a part of the best practices for open-source software. See trixi-framework/Trixi.jl#1884

You need to enable "Private vulnerability reporting" in the settings of this repository (Settings -> Code security and analysis, first item for me).

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 52.48%. Comparing base (bbe7f9b) to head (9e647d0).

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #23   +/-   ##
=======================================
  Coverage   52.48%   52.48%           
=======================================
  Files           3        3           
  Lines        1427     1427           
=======================================
  Hits          749      749           
  Misses        678      678           

Flag	Coverage Δ
unittests	52.48% <ø> (ø)

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