Allow .txt file for "credits" or "readme" as file type on resource page.

Currently you can only attach files by labeling them as the resource itself, or as the training data. I think it would be very helpful and relatively easy to implement allowing including a text file for “credits”, which would greatly help people using CC-By images in their training comply with license terms (currently the best way to do this is to include it in the description, which is very suboptimal). While at it, it would be trivial to expand to also allowing txt files set as “readme” as well (generally things like prompt instructions and recommendations are included in the description, but I’m sure it would be useful to a lot of model makers. Maybe even option to flat out DL the description of a resource as a .txt instead of forcing a copy-paste job to preserve that information.)