Add whatsnew entry

Author: DGrady

doc/source/whatsnew/v0.21.0.txt

@@ -127,7 +127,7 @@ Sparse
 Reshaping
 ^^^^^^^^^
 
-
+- `{arg,idx}{min,max}` on Series, DataFrame, and GroupBy objects work correctly with float data that contains infinite values (:issue:`13595`), and with string data as long as it does not have any missing values.
 
 Numeric
 ^^^^^^^