Add polish translation

CONTRIBUTING.md

@@ -69,6 +69,7 @@ Italiano   | [progit/progit2-it](https://github.com/progit/progit2-it)
 日本語   | [progit/progit2-ja](https://github.com/progit/progit2-ja)
 한국어   | [progit/progit2-ko](https://github.com/progit/progit2-ko)
 Nederlands | [progit/progit2-nl](https://github.com/progit/progit2-nl)
+Polski | [progit2-pl/progit2-pl](https://github.com/progit2-pl/progit2-pl)
 Português (Brasil) | [progit2-pt-br/progit2](https://github.com/progit2-pt-br/progit2)
 Русский   | [progit/progit2-ru](https://github.com/progit/progit2-ru)
 Slovenščina  | [progit/progit2-sl](https://github.com/progit/progit2-sl)