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

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.59%. Comparing base (f2e2d6f) to head (9ce01e7).

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #19   +/-   ##
=======================================
  Coverage   94.59%   94.59%           
=======================================
  Files           4        4           
  Lines          74       74           
=======================================
  Hits           70       70           
  Misses          4        4           

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[coveralls]

Pull Request Test Coverage Report for Build 8410472254

Warning: This coverage report may be inaccurate.

This pull request's base commit is no longer the HEAD commit of its target branch. This means it includes changes from outside the original pull request, including, potentially, unrelated coverage changes.

• For more information on this, see Tracking coverage changes with pull request builds.
• To avoid this issue with future PRs, see these Recommended CI Configurations.
• For a quick fix, rebase this PR at GitHub. Your next report should be accurate.

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 94.595%

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Totals
Change from base Build 8296741263:	0.0%
Covered Lines:	70
Relevant Lines:	74

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💛 - Coveralls