:busts_in_silhouette: update/fix contributors

[techouse]

• fixed .all-contributors
• added @Guldem to .all-contributors and README.md
• added Code of Conduct based on https://www.contributor-covenant.org/version/2/1/code_of_conduct.html
• added Contributing guidelines

I used my email in the Code of Conduct document. I can replace it with another, if someone wants to take this over.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 93.05%. Comparing base (bb52f38) to head (29a5207).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## develop #618 +/- ## ======================================== Coverage 93.05% 93.05% ======================================== Files 22 22 Lines 504 504 ======================================== Hits 469 469 Misses 35 35 ```

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[Guldem]

Cool! Thanks for adding me :)