docs(secondhalf): instructions upgrade

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

subjects/secondhalf/README.md

@@ -0,0 +1,41 @@
+## secondhalf
+
+### Instructions
+
+Write a function `SecondHalf()` that receives a slice of `int` and returns another slice of `int` with the second half of the values.
+
+- If the length is odd, round it up.
+
+### Expected function
+
+```go
+func SecondHalf(slice []int) []int {
+
+}
+```
+
+### Usage
+
+Here is a possible program to test your function:
+
+```go
+package main
+
+import (
+	"fmt"
+	"piscine"
+)
+
+func main() {
+	fmt.Println(piscine.SecondHalf([]int{1, 2, 3, 4, 5, 6, 7, 8, 9, 10}))
+	fmt.Println(piscine.SecondHalf([]int{1, 2, 3}))
+}
+```
+
+And its output:
+
+```console
+$ go run . | cat -e
+[6 7 8 9 10]
+[2 3]
+```