Add CSV file for dataset viewer compatibility

.gitattributes

@@ -57,3 +57,4 @@ saved_model/**/* filter=lfs diff=lfs merge=lfs -text
 # Video files - compressed
 *.mp4 filter=lfs diff=lfs merge=lfs -text
 *.webm filter=lfs diff=lfs merge=lfs -text
+aozorabunko_with_difficulty_5k.csv filter=lfs diff=lfs merge=lfs -text

aozorabunko_with_difficulty_5k.csv

@@ -0,0 +1,3 @@
+version https://git-lfs.github.com/spec/v1
+oid sha256:39fae304bc0abc7931ace41b8c7a0e2e31b0de5fd387844a732b83b79b026848
+size 183642996