docs: add .lazytest guidance

[maelle]

Fix #12

[codecov]

Codecov Report

Merging #23 (0ca63de) into main (5c87ca0) will not change coverage.
The diff coverage is n/a.

❗ Current head 0ca63de differs from pull request most recent head 797260a. Consider uploading reports for the commit 797260a to get more accurate results

@@           Coverage Diff           @@
##             main      #23   +/-   ##
=======================================
  Coverage   77.50%   77.50%           
=======================================
  Files           3        3           
  Lines         120      120           
=======================================
  Hits           93       93           
  Misses         27       27           

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[krlmlr]

Thanks!