docs: Add readme for sum subject

subjects/sum/README.md

@@ -0,0 +1,42 @@
+## sum
+
+### Instructions
+
+Write a function that takes two single digit numbers as a string and returns the sum as an int. If one of the arguments is not a single digit number, return 0;
+
+### Expected function
+
+```go
+func Sum(a,b string) int {
+
+}
+```
+
+### Usage
+
+Here is a possible program to test your function :
+
+```go
+package main
+
+import (
+    "piscine"
+    "fmt"
+)
+
+func main() {
+	fmt.Println(piscine.Sum("1", "2"))
+	fmt.Println(piscine.Sum("-4", "0"))
+	fmt.Println(piscine.Sum("7", "-3"))
+}
+```
+
+And its output :
+
+```console
+$ go run . | cat -e
+3$
+-4$
+4$
+$
+```