Add contribution guideline

[raimon49]

Add CONTRIBUTING.md file for contributors.

See also:

• Setting guidelines for repository contributors - User Documentation

[codecov]

Codecov Report

Merging #23 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #23   +/-   ##
=======================================
  Coverage   97.59%   97.59%           
=======================================
  Files           1        1           
  Lines         166      166           
=======================================
  Hits          162      162           
  Misses          4        4

---

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[codecov]

Codecov Report

Merging #23 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #23   +/-   ##
=======================================
  Coverage   97.59%   97.59%           
=======================================
  Files           1        1           
  Lines         166      166           
=======================================
  Hits          162      162           
  Misses          4        4

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 950fb17...e86ff37. Read the comment docs.