Add contributing file to root

[kwinkunks]

Moved the contribution guidelines from the docs to the root folder. Included the root-level file in the docs, so no change there. Resolves #421.

This should not require the new file to be pointed at by the contributing 'help' in GitHub, because according to the docs

If a repository contains more than one CONTRIBUTING file, then the file shown in links is chosen from locations in the following order: the .github directory, then the repository's root directory, and finally the docs directory.

I actually did not know that about the docs directory, but in any case Equinor's guidelines ask for the file in the root dir.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (c8c385b) 98.98% compared to head (1778d5d) 98.98%.

❗ Current head 1778d5d differs from pull request most recent head 6fc8906. Consider uploading reports for the commit 6fc8906 to get more accurate results

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #423   +/-   ##
=======================================
  Coverage   98.98%   98.98%           
=======================================
  Files          16       16           
  Lines        2558     2558           
=======================================
  Hits         2532     2532           
  Misses         26       26           

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[alifbe]

LGTM 👍