add contribution guidelines

[jurekbarth]

No description provided.

[codecov]

Codecov Report

Merging #108 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #108   +/-   ##
=======================================
  Coverage   83.33%   83.33%           
=======================================
  Files           5        5           
  Lines          48       48           
  Branches        6        6           
=======================================
  Hits           40       40           
  Misses          7        7           
  Partials        1        1

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