docs(printfirsthalf): instructions upgrade

- fix exercise header
- upgrade instructions
- add missig import 'fmt'/'piscine'
- fix console
- fix white-spaces and indentation

subjects/printfirsthalf/README.md

@@ -0,0 +1,45 @@
+## printfirsthalf
+
+### Instructions
+
+Write a function called `PrintFirstHalf()` that takes a `string` as an argument and prints the first half of a `string` followed by a newline `\n`.
+
+- If the length of the `string` is odd, round it down.
+- If the `string` is empty, return `Invalid Output` followed by a newline `\n`.
+- If the `string` length is equal to one, return the `string` followed by a newline `\n`.
+
+### Expected function
+
+```go
+func PrintFirstHalf(str string) string {
+
+}
+```
+
+### Usage
+
+Here is a possible program to test your function:
+
+```go
+package main
+
+import (
+	"fmt"
+	"piscine"
+)
+
+func main() {
+	fmt.Print(piscine.PrintFirstHalf("This is the 1st halfThis is the 2nd half"))
+	fmt.Print(piscine.PrintFirstHalf(""))
+	fmt.Print(piscine.PrintFirstHalf("Hello World"))
+}
+```
+
+And its output:
+
+```console
+$ go run . | cat -e
+This is the 1st half$
+Invalid Output$
+Hello$
+```