Added Norwegian Nynorsk translation in index.no_NN.json, renamed index.nb_NO.json to index.no_NB.json as instructed in README.md.

The Norwegian Nynorsk translation was done by hand by referencing the English original. Additionally, the Norwegian Bokmål version ended up feeling a little Google Translate-ish after comparing it to the Nynorsk version, so I did a few edits in there again.