chore: add CONTRIBUTING.md

[gian1200]

Add a basic/generic CONTRIBUTING.md. Based on #195 (comment).

Feel free to update it as needed

[TimothyJones]

Thank you so much! This is great!

[codecov-commenter]

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Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 92.80%. Comparing base (7ebdacd) to head (973c342).
Report is 29 commits behind head on master.

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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #198      +/-   ##
==========================================
- Coverage   94.11%   92.80%   -1.32%     
==========================================
  Files          26       29       +3     
  Lines         493      542      +49     
==========================================
+ Hits          464      503      +39     
- Misses         29       39      +10     

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