Adds a guide to contributing documents

[sinisaos]

Related to #459

[codecov-commenter]

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Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.04%. Comparing base (1157c12) to head (ba36810).
⚠️ Report is 48 commits behind head on master.
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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #461      +/-   ##
==========================================
+ Coverage   93.42%   94.04%   +0.62%     
==========================================
  Files           5        5              
  Lines         365      403      +38     
==========================================
+ Hits          341      379      +38     
  Misses         24       24              

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