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 94.55%. Comparing base (591fe75) to head (493a266).

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #45   +/-   ##
=======================================
  Coverage   94.55%   94.55%           
=======================================
  Files          31       31           
  Lines        2810     2810           
=======================================
  Hits         2657     2657           
  Misses        153      153           

Flag	Coverage Δ
unittests	94.55% <ø> (ø)

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[andrewwinters5000]

Thanks! I have activated the "Private vulnerability reporting".