Update FlatMap docs to talk about nil case (#314)

README.md

@@ -320,7 +320,7 @@ matching := lo.FilterMap([]string{"cpu", "gpu", "mouse", "keyboard"}, func(x str
 
 ### FlatMap
 
-Manipulates a slice and transforms and flattens it to a slice of another type.
+Manipulates a slice and transforms and flattens it to a slice of another type. The transform function can either return a slice or a `nil`, and in the `nil` case no value is added to the final slice.
 
 ```go
 lo.FlatMap([]int{0, 1, 2}, func(x int, _ int) []string {