Adding new translation - Polish (pl_PL)

Hello!

We would like to contribute to the awesome initiative of nodeschool.io by preparing Polish translation of the site. It would be a first step for us, because we'd like to host some events in Poland, in the Silesia region (so probably our next step will involve creating an issue [here](https://github.com/nodeschool/organizers/issues)).

Feel free to comment and give us feedback, if we missed something here. :wink: