Add information that pyi files are used in runtime when use attach_stub

[Czaki]

In this PR I try to clarify that attach_stub impacts not only type checking for IDE but also runtime.

closes #38

[codecov]

Codecov Report

Merging #47 (563064d) into main (acbce79) will not change coverage.
The diff coverage is n/a.

❗ Current head 563064d differs from pull request most recent head f178c63. Consider uploading reports for the commit f178c63 to get more accurate results

@@           Coverage Diff           @@
##             main      #47   +/-   ##
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  Coverage   90.53%   90.53%           
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  Files           4        4           
  Lines         169      169           
=======================================
  Hits          153      153           
  Misses         16       16           

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[stefanv]

Thanks @Czaki!