Add licensing information

README.md

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 # PonyPaper
 A live wallpaper for android with characters from My Little Pony: Friendship is Magic
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+## Licensing
+All artwork was created by contributors to the Desktop Ponies team (who have a [DeviantArt group](http://desktop-pony-team.deviantart.com/) and a [source repository](https://github.com/RoosterDragon/Desktop-Ponies)). It is licensed under [Creative Commons Attribution-NonCommercial-ShareAlike 3.0 Unported (CC BY-NC-SA 3.0)](http://creativecommons.org/licenses/by-nc-sa/3.0/).
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+The source code is available under the same license.
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+This means that anyone is free to share and modify this project, provided you give credit, do not use it commercially and allow others to use your modifications under the same terms.