Add link to Polish translation (wesm#130)

* update readme

added Polish

* Update README.md

Small change with resource for Polish.

README.md

@@ -17,6 +17,7 @@ reorganized book materials on the [`1st-edition` branch][2].
 # Translations
 
 * [Chinese][3] by Xu Liang
+* [Polish][4] by Michal Biesiada
 
 ## IPython Notebooks:
 
@@ -45,4 +46,5 @@ above, is released under the [MIT license](LICENSE-CODE). Read more at the
 
 [1]: http://amzn.to/2vvBijB
 [2]: https://github.com/wesm/pydata-book/tree/1st-edition
-[3]: https://github.com/BrambleXu/pydata-notebook
+[3]: https://github.com/BrambleXu/pydata-notebook
+[4]: https://github.com/mbiesiad/pydata-book/tree/pl_PL