Subpackage readme

[hakkelt]

• Add Readme files as suggested in a comment in New package: DSPOperators v0.1.0 JuliaRegistries/General#144518 (comment)
• Remove CI and coverage badge from the documentation front page because they were configured to display the state of the master branch, which is incorrect in case of a versioned documentation page. Instead of fixing it, I think it is better to be removed.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 87.60%. Comparing base (ea5d8c2) to head (0d4ecf7).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master      #36   +/-   ##
=======================================
  Coverage   87.60%   87.60%           
=======================================
  Files          45       45           
  Lines        3026     3026           
=======================================
  Hits         2651     2651           
  Misses        375      375           

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