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]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 70.57%. Comparing base (1ca7fff) to head (4ec4f60).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #477   +/-   ##
=======================================
  Coverage   70.57%   70.57%           
=======================================
  Files          66       66           
  Lines        3827     3827           
=======================================
  Hits         2701     2701           
  Misses       1126     1126           

Flag	Coverage Δ
unit	70.57% <ø> (ø)

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