update readme logo with text.

[kellertuer]

No description provided.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.90%. Comparing base (a83f0bf) to head (db788cc).
Report is 2 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #412   +/-   ##
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  Coverage   99.90%   99.90%           
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  Files          78       78           
  Lines        8271     8271           
=======================================
  Hits         8263     8263           
  Misses          8        8           

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[mateuszbaran]

I just thought, maybe the lines in the logo could use Julia colors instead of being black? There are two groups of three lines, which would fit nicely.

[kellertuer]

In the long run that is one of the plans why I started to redo the logo in Julia; for now this is the logo file generated with the Matlab code, where the contours are computed in angles (as if that were part of $\mathbb R^2$); in the Matlab code I am not able to choose individual colours. Also Julia colors might require the colormap to be adapted as well, so the logo still looks nice.