update readme with index info

weed/filer/arangodb/readme.md

@@ -41,12 +41,10 @@ should there be one collection per bucket? would make deleting a bucket instant
 
 ## comparison
 
-arangodb uses rocksdb in the background, so i am assuming things run in log time
-
-i am not sure how the prefix query scales compared to the recursive calls that some other stores do for folder deletion
-might need to change that.
-
-
 ok, so if https://www.arangodb.com/docs/stable/indexing-index-basics.html#persistent-index is correct
+
 it should be log time to the number of files in the directory
+
 and constant time if you have full directory + file
+
+deleting a folder should be log time to number of folders + files that need to be deleted