docs(gcd): update subject to require a function instead of a program

- add main.go to test the function in exam mode

subjects/gcd/README.md

@@ -2,26 +2,46 @@
 
 ### Instructions
 
-Write a program that takes two `string` representing two strictly positive integers that fit in an `int`.
+Write a function that takes two `uint` representing two strictly positive integers and returns their greatest common divisor.
+If any of the input numbers is 0, the function should return 0.
 
-The program displays their greatest common divisor followed by a newline (`'\n'`).
+> [In mathematics, the greatest common divisor (GCD) of two or more integers, which are not all zero, is the largest positive integer that divides each of the integers.](https://en.wikipedia.org/wiki/Greatest_common_divisor)
 
-If the number of arguments is different from 2, the program displays nothing.
+### Expected function
 
-All arguments tested will be positive `int` values.
+```go
+func Gcd(a, b uint) uint {
+
+}
+```
 
 ### Usage
 
+Here is a possible program to test your function:
+
+```go
+package main
+
+import (
+	"fmt"
+	"piscine"
+)
+
+func main() {
+	fmt.Println(piscine.Gcd(42, 10))
+	fmt.Println(piscine.Gcd(42, 12))
+	fmt.Println(piscine.Gcd(14, 77))
+	fmt.Println(piscine.Gcd(17, 3))
+}
+```
+
+And its output :
+
 ```console
-$ go run . 42 10 | cat -e
-2$
-$ go run . 42 12
+$ go run .
+2
 6
-$ go run . 14 77
 7
-$ go run . 17 3
 1
-$ go run .
-$ go run . 50 12 4
 $
 ```

subjects/gcd/main.go

@@ -0,0 +1,13 @@
+package main
+
+import (
+	"fmt"
+	"piscine"
+)
+
+func main() {
+	fmt.Println(piscine.Gcd(42, 10))
+	fmt.Println(piscine.Gcd(42, 12))
+	fmt.Println(piscine.Gcd(14, 77))
+	fmt.Println(piscine.Gcd(17, 3))
+}