Add polish translation

[decpio]

This is polish translation. I'm native in that language, quality is good. Please append that file to repo.

[vaenow]

@decpio Thanks for your translation.

Please add a line configuration in plugin.xml for translation

like this: https://github.com/vaenow/cordova-plugin-app-update/pull/57#issuecomment-337468851

[vaenow]

ref https://github.com/vaenow/cordova-plugin-app-update/pull/71

[vaenow]

Thank you @decpio