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 98.85%. Comparing base (9352a60) to head (c096312).

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #48   +/-   ##
=======================================
  Coverage   98.85%   98.85%           
=======================================
  Files          20       20           
  Lines        1838     1838           
=======================================
  Hits         1817     1817           
  Misses         21       21           

Flag	Coverage Δ
unittests	98.85% <ø> (ø)

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[coveralls]

coverage: 98.857%. remained the same
when pulling c096312 on ranocha-patch-1
into 9352a60 on main.

[andrewwinters5000]

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