Update README.md

[janfb]

• update discord link
• fix tests badge

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 78.62%. Comparing base (ecfcec5) to head (e17469a).
Report is 1 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #1327       +/-   ##
===========================================
- Coverage   89.39%   78.62%   -10.77%     
===========================================
  Files         118      118               
  Lines        8709     8716        +7     
===========================================
- Hits         7785     6853      -932     
- Misses        924     1863      +939     

Flag	Coverage Δ
unittests	78.62% <ø> (-10.77%)	⬇️

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see 32 files with indirect coverage changes

[michaeldeistler]

Thanks!