docs(concatslice): fix subject

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

subjects/concatslice/README.md

@@ -0,0 +1,41 @@
+## concatslice
+
+### Instructions
+
+Write a function `ConcatSlice()` that takes two slices of integers as arguments and returns the concatenation of the two slices.
+
+### Expected function
+
+```go
+func ConcatSlice(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.ConcatSlice([]int{1, 2, 3}, []int{4, 5, 6}))
+	fmt.Println(piscine.ConcatSlice([]int{}, []int{4, 5, 6, 7, 8, 9}))
+	fmt.Println(piscine.ConcatSlice([]int{1, 2, 3}, []int{}))
+}
+```
+
+And its output:
+
+```console
+$ go run .
+[1 2 3 4 5 6]
+[4 5 6 7 8 9]
+[1 2 3]
+```