Update README.md

[creisle]

Fix badge link so it points to the correct "master" branch which is actually "main"

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 87.86%. Comparing base (79c350c) to head (2ef2c5a).

Additional details and impacted files

@@           Coverage Diff            @@
##           develop     #126   +/-   ##
========================================
  Coverage    87.86%   87.86%           
========================================
  Files           18       18           
  Lines         2636     2636           
========================================
  Hits          2316     2316           
  Misses         320      320           

Flag	Coverage Δ
unittests	87.86% <ø> (ø)

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