you can store the content of the text file in the note field of a
card. This card could be without any other content but the note.
The interface of the application is not exactly tuned for this kind of
usage, but it should work.
The note field is usually quite short when displayed, but you can
easily make it taller simply dragging the handler on its bottom border
(both when reading and editing it).
Hope this helps.
Best regards,
Giulio Cesare