add contribution guide

[Moelf]

fix #170

[codecov]

Codecov Report

Merging #172 (060dded) into master (f7437dc) will not change coverage.
The diff coverage is n/a.

❗ Current head 060dded differs from pull request most recent head 683cd2c. Consider uploading reports for the commit 683cd2c to get more accurate results

@@           Coverage Diff           @@
##           master     #172   +/-   ##
=======================================
  Coverage   90.14%   90.14%           
=======================================
  Files          11       11           
  Lines        1604     1604           
=======================================
  Hits         1446     1446           
  Misses        158      158           

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