add badge and how to cite

[xiki-tempula]

Fix #415

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.80%. Comparing base (968d4f2) to head (f87f583).
Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #426   +/-   ##
=======================================
  Coverage   98.80%   98.80%           
=======================================
  Files          28       28           
  Lines        2008     2008           
  Branches      356      356           
=======================================
  Hits         1984     1984           
  Misses          2        2           
  Partials       22       22           

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[orbeckst]

Can you please also update the JOSS paper in citation.cff — issue & page number. Thanks.

[orbeckst]

great, thank you!