update readme

added Polish ver. info

Author: mbiesiad

readme.md

@@ -32,6 +32,7 @@ Start by understanding the ubiquitous testing practices that are the foundation
 
 - 🇨🇳[Chinese](readme-zh-CN.md) - courtesy of [Yves yao](https://github.com/yvesyao)
 - 🇰🇷[Korean](readme.kr.md) - courtesy of [Rain Byun](https://github.com/ragubyun)
+- 🇵🇱[Polish](readme.pl.md) - courtesy of [Michal Biesiada](https://github.com/mbiesiad)
 - Want to translate to your own language? please open an issue 💜
 
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