Add `pnpm` adapter usage to README.md

[SergeAstapov]

This is follow up for #787 that makes discoverability of the feature easier.

[codecov[bot]]

Codecov Report

Merging #839 (1c9cb87) into master (17592f8) will not change coverage. The diff coverage is n/a.

:exclamation: Current head 1c9cb87 differs from pull request most recent head b2dfdc7. Consider uploading reports for the commit b2dfdc7 to get more accurate results

@@           Coverage Diff           @@
##           master     #839   +/-   ##
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  Coverage   94.36%   94.36%           
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  Files          17       17           
  Lines         550      550           
=======================================
  Hits          519      519           
  Misses         31       31           

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