Add CONTRIBUTING.md

[dereuromark]

Summary

• Add CONTRIBUTING.md with development guidelines for potential contributors

Covers:

• Development setup instructions
• Code style requirements (PSR-12 + PHPCollective ruleset)
• Running tests and static analysis
• PR process
• Project structure overview
• Issue reporting guidelines

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.92%. Comparing base (02c1ce4) to head (2465001).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##             master      #38   +/-   ##
=========================================
  Coverage     86.92%   86.92%           
  Complexity     1009     1009           
=========================================
  Files            29       29           
  Lines          2340     2340           
=========================================
  Hits           2034     2034           
  Misses          306      306           

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