Add licensing information regarding Zpix font

[pdinklag]

The license of the Zpix font, which is used for the Chinese localization, requires payment of a license fee for commercial products. This must be noted in the documentation, because the MinecraftStats license (CC BY-SA 4.0) explicitly allows commerical use. People who wish to use MinecraftStats commercially will have to pay the license fee to the Zpix author, or remove the font from their installation.