Update the Readme for the dataframe directory (#4757)

* update dataframe readme

* update df readme

crates/nu-command/src/dataframe/README.md

@@ -1,3 +1,12 @@
-# nu-dataframe
+# Dataframe
 
-The nu-dataframe crate holds the definitions of the dataframe structures and commands
+This dataframe directory holds all of the definitions of the dataframe data structures and commands.
+
+There are three sections of commands:
+
+* [eager](./eager)
+* [series](./series)
+* [values](./values)
+
+For more details see the
+[Nushell book section on dataframes](https://www.nushell.sh/book/dataframes.html)