there's an opensubtitles plugin to search for subs and download them; you can also manually upload them by going to a library item, clicking the three dots, an selecting edit subtitles. you can search by clicking the magnifying glass or add your own by clicking the plus.
you can also just drop srts in the folder containing your item named like <name of media file>.en.srt.
haven't found a way to automatically download subs though :/