Add SECURITY.md

[arkid15r]

Proposed change

Add GitHub repository security policy.

Type of change

• New country/market holidays support (thank you!)
• Supported country/market holidays update (calendar discrepancy fix, localization)
• Existing code/documentation/test/process quality improvement (best practice, cleanup, refactoring, optimization)
• Dependency update (version deprecation/pin/upgrade)
• Bugfix (non-breaking change which fixes an issue)
• Breaking change (a code change causing existing functionality to break)
• New feature (new holidays functionality in general)

Checklist

• I've followed the contributing guidelines
• I've successfully run make check, all checks and tests are green

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (93b3ca9) to head (46793f8).
Report is 1 commits behind head on dev.

Additional details and impacted files

@@            Coverage Diff            @@
##               dev     #2142   +/-   ##
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  Coverage   100.00%   100.00%           
=========================================
  Files          194       194           
  Lines        11716     11716           
  Branches      1771      1771           
=========================================
  Hits         11716     11716           

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[KJhellico]

LGTM.