Add endonyms and iso639-3 codes

[Oktogazh]

Hi, thank you for this work.
I have made a little list of endonyms (native name of a language in its own script) based on this page with the related iso639-3 codes (and glottocodes, but that's easier to map once we have the iso codes).
I think that it would be nice to add them to the repo, but I am not sure how, so I leave an issue here. If you ask, I'll send you the json file.

Cheers

[mattcg]

Hi, thank you for suggesting this! I think they should really be added to the original IANA source. Have you suggested it to them?