docs(revconcatalternate): fix instructions

- format exercise title
- upgrade instructions
- add missing import 'piscine'
- fix white-spaces
- fix output

subjects/revconcatalternate/README.md

@@ -0,0 +1,44 @@
+## revconcatalternate
+
+### Instructions
+
+Write a function `RevConcatAlternate()` that receives two slices of `int` as arguments and returns a new slice with alternated values of each slice in reverse order.
+- The input slices can have different lengths.
+- The new slice should start with an element of the largest slice first.
+- If the slices are of equal length, the new slice should start with an element of the first slice.
+
+### Expected function
+
+```go
+func RevConcatAlternate(slice1,slice2 []int) []int {
+
+}
+```
+
+### Usage
+
+Here is a possible program to test your function:
+
+```go
+package main
+
+import (
+	"fmt"
+	"piscine"
+)
+
+func main() {
+	fmt.Println(piscine.RevConcatAlternate([]int{1, 2, 3}, []int{4, 5, 6}))
+	fmt.Println(piscine.RevConcatAlternate([]int{1, 2, 3}, []int{4, 5, 6, 7, 8, 9}))
+	fmt.Println(piscine.RevConcatAlternate([]int{1, 2, 3}, []int{}))
+}
+```
+
+And its output:
+
+```console
+$ go run .
+[3 6 2 5 1 4]
+[9 3 8 2 7 1 6 5 4]
+[3 2 1]
+```