docs: add CONTRIBUTING.md as a best-practices reference for contributors

[rmiki-dev]

CONTRIBUTING.md file to guide new contributors through the development workflow, commit requirements, and PR process for the tar-diff project.

A "Contributing to tar-diff" section has also been added to README.md directing users to the guide

The key sections in this PR include:

• Prerequisites and setup
• Development workflow
• DCO signoff and commit signing requirements
• Conventional commits format with examples
• Testing and validation instructions
• Code quality standards and PR process
• Merge vs squash strategy for clean git history

Co-Authored-By: Claude Sonnet 4.5 noreply@anthropic.com

[gemini-code-assist]

Code Review

This pull request introduces a comprehensive CONTRIBUTING.md file outlining the project's development workflow, commit standards, and testing procedures. Feedback was provided to correct a typo and remove trailing whitespace in the introduction, as well as to update the minimum Go version requirement to 1.26 to align with the project's go.mod file.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 79.37%. Comparing base (afb00d4) to head (2faade9).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #92      +/-   ##
==========================================
+ Coverage   72.87%   79.37%   +6.49%     
==========================================
  Files          10       10              
  Lines        1412     1115     -297     
==========================================
- Hits         1029      885     -144     
+ Misses        271      133     -138     
+ Partials      112       97      -15     

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