Yes, Fountain supports transcripts. When publishing an episode in Fountain, have two options:
Fountain will start generating a transcript for your episode as soon as you've uploaded your audio or video. After publishing the episode, the transcript will be included in your feed and will be available to listeners in supporting apps.
If you already have a transcript from a third-party service, you have the option to upload an SRT file. Just check the box next to "I want to upload my own transcript" and choose the file.