docs(fishandchips): upgrade subject instructions

subjects/fishandchips/README.md

@@ -0,0 +1,49 @@
+## fishandchips
+
+### Instructions
+
+Write a function called `FishAndChips()` that takes an `int` and returns a `string`.
+
+- If the number is divisible by 2, print `fish` followed by a newline `\n`.
+- If the number is divisible by 3, print `chips` followed by a newline `\n`.
+- If the number is divisible by 2 and 3, print `fish and chips` followed by a newline `\n`.
+- If the number is negative return `error: number is negative` followed by a newline `\n`.
+- If the number is non divisible return `error: non divisible` followed by a newline `\n`.
+
+### Expected function
+
+```go
+func FishAndChips(n int) string {
+
+}
+```
+
+### Usage
+
+Here is a possible program to test your function:
+
+```go
+package main
+
+import (
+	"fmt"
+	"piscine"
+)
+
+func main() {
+	fmt.Println(piscine.FishAndChips(4))
+	fmt.Println(piscine.FishAndChips(9))
+	fmt.Println(piscine.FishAndChips(6))
+}
+```
+
+And its output:
+
+```console
+fish$
+$
+chips$
+$
+fish and chips$
+$
+```