DEV-3184 add(docs):add subject of concatalter

subjects/concatalernate/README.md

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+## Concat-Alternate
+
+
+### Instructions
+
+- Write a function that receives two slices of int as arguments and returns a new one with the result of alternating the values of each.
+- The input slices may be of different lengths.
+- The new slice should contain the elements of the larger slice first and then the elements of the smaller slice.
+- If the slices are of equal length, the new slice should contain the elements of the first slice first and then the elements of the second slice.
+- The new slice should be in the same order as the input slices.
+
+### Expected function
+
+```go
+func ConcatAlternate(slice1,slice2 []int) []int {
+    
+}
+```
+
+### Usage
+
+Here is a possible program to test your function :
+
+```go
+package main
+
+import (
+	"fmt"
+)
+
+func main() {
+	fmt.Println(ConcatAlternate([]int{1,2,3},[]int{4,5,6}))
+    fmt.Println(ConcatAlternate([]int{1,2,3},[]int{4,5,6,7,8,9}))
+    fmt.Println(ConcatAlternate([]int{1,2,3},[]int{}))
+}
+```
+
+And its output :
+
+```console
+$ go run .
+[1, 4, 2, 5, 3, 6]
+[1, 4, 2, 5, 3, 6, 7, 8, 9]
+[1, 2, 3]
+```