docs(checknumber): added subject

- it was removed by mistake before

subjects/checknumber/README.md

@@ -0,0 +1,37 @@
+## checknumber
+
+### Instructions
+
+Write a function that takes a `string` as an argument and returns `true` if the string contains any number, otherwise return `false`.
+
+### Expected function
+
+```go
+func CheckNumber(arg string) bool {
+}
+```
+
+### Usage
+
+Here is a possible program to test your function:
+
+```go
+package main
+import (
+	"fmt"
+	"piscine"
+)
+func main() {
+	fmt.Println(piscine.CheckNumber("Hello"))
+	fmt.Println(piscine.CheckNumber("Hello1"))
+}
+```
+
+And its output:
+
+```console
+$ go run .
+false
+true
+$
+```