Update README.md to new version.

[tomoverlund]

This Pull Request fixes/closes #4253.

It changes the following:

• README.md: Change 0.19.0 to 0.20.0

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 53.08%. Comparing base (6ddc2b4) to head (dcb3bf0).
Report is 414 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4254      +/-   ##
==========================================
+ Coverage   47.24%   53.08%   +5.84%     
==========================================
  Files         476      486      +10     
  Lines       46892    50130    +3238     
==========================================
+ Hits        22154    26614    +4460     
+ Misses      24738    23516    -1222     

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[HalidOdat]

Looks good to me! Thanks for the contribution @tomoverlund :)