Update README.md

README.md

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 # indexed-transformers
-Atkey Indexed Monad Transformers
+
+An [Atkey indexed monad]
+(https://bentnib.org/paramnotions-jfp.pdf)
+is a `Functor` [enriched category]
+(https://ncatlab.org/nlab/show/enriched+category).
+An indexed monad transformer transforms a `Monad` into an indexed monad.