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 74.19%. Comparing base (85818b3) to head (4676d25).

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #93   +/-   ##
=======================================
  Coverage   74.19%   74.19%           
=======================================
  Files          67       67           
  Lines       10155    10155           
  Branches        2        2           
=======================================
  Hits         7534     7534           
  Misses       2621     2621           

Flag	Coverage Δ
unittests	74.19% <ø> (ø)

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[andrewwinters5000]

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