docs(halfslice): fix subject

- upgrade intructions
- add missing import 'piscine'
- fix console and its output
- fix white-spaces

subjects/halfslice/README.md

@@ -0,0 +1,39 @@
+## halfslice
+
+### Instructions
+
+Write a function `HalfSlice()` that receives a slice of `int` and returns another slice of `int` with the first half of the values. If the length is odd, round it up.
+
+### Expected function
+
+```go
+func HalfSlice(slice []int) []int {
+
+}
+```
+
+### Usage
+
+Here is a possible program to test your function:
+
+```go
+package main
+
+import (
+	"fmt"
+	"piscine"
+)
+
+func main() {
+	fmt.Println(piscine.HalfSlice([]int{1, 2, 3, 4, 5, 6, 7, 8, 9, 10}))
+	fmt.Println(piscine.HalfSlice([]int{1, 2, 3}))
+}
+```
+
+And its output:
+
+```console
+$ go run . | cat -e
+[1 2 3 4 5]
+[1 2]
+```