Add readme to crepe_ft

crepe_ft/README.md

+This folder contains the pretrained weights used in benchmarking model performances.
+File naming has the following structure : `model_{only/omit}_{taxa}.pth'
+`omit' files are models trained on all taxa except the one specified after.
+`only' files are models trained on a single taxa, in a 5-fold manner (the following number specifies the fold kept for testing).
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