fix: update documentation for new sliceutils.FlatMap function (#44)

* fix: update documentation for new sliceutils.FlatMap function

* Update docs/sliceutils/flatMap.md

Co-authored-by: Na'aman Hirschfeld <nhirschfeld@gmail.com>

---------

Co-authored-by: Na'aman Hirschfeld <nhirschfeld@gmail.com>

Author: deansg

docs/sliceutils/flatMap.md

@@ -0,0 +1,24 @@
+# FlatMap
+
+`func FlatMap[T any, R any](slice []T, mapper func(value T, index int, slice []T) []R) []R`
+
+FlatMap receives a slice of type T, executes the passed in slice-mapper function for each element in the slice, and
+returns a flattened slice containing all the elements from all the mapped slices.
+The function is passed the current element, the current index and the slice itself as function arguments.
+
+
+```go
+package main
+
+import (
+	"github.com/Goldziher/go-utils/sliceutils"
+)
+
+func main() {
+	items := []int{1, 2, 3, 4}
+
+	flatMapped := sliceutils.FlatMap(items, func(value int, index int, slice []int) []int {
+		return []int{value, value * 2}
+	}) // []int{1, 2, 2, 4, 3, 6, 4, 8}
+}
+```