feat: Adding docs via CONTRIBUTING.md

[malteos]

This PR adds infos on running tests as documentation.

• Fixes add to doco #56
• The common practice for developer guidelines etc. is to have them in a CONTRIBUTING.md.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.89%. Comparing base (b1fea6c) to head (f4674f1).

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #59   +/-   ##
=======================================
  Coverage   95.89%   95.89%           
=======================================
  Files           7        7           
  Lines         876      876           
=======================================
  Hits          840      840           
  Misses         36       36           

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.