Update movies.md

[seaniedan]

trafficstars

There was no info about how to name subtitles https://jellyfin.org/docs/general/server/media/subtitles.html existed but is no longer there.

Would like to add: if you want subtitle to be the default: Movie.default.srt If you want to define the subtitle's language tag:(such as English) Movie.eng.default.srt

A full list of language codes is available at https://www.loc.gov/standards/iso639-2/php/code_list.php

[Shadowghost]

External file documentation is located at https://jellyfin.org/docs/general/server/media/external-files.html