feat(hashcode): add README.md for new go exercise hashcode

subjects/hashcode/README.md

@@ -0,0 +1,45 @@
+## hashcode
+
+### Instructions
+
+Write a function called `HashCode()` that takes a `string` as an argument and returns a new **hashed** `string`.
+
+- Hash equation: (ASCII of current character + size of the string) % 127, so it can be in the limit of the ASCII size '127'.
+
+- If the final number gives an unprintable character, add 33.
+
+
+### Expected function
+
+```go
+func HashCode(dec string) string {
+}
+```
+
+### Usage
+
+Here is a possible program to test your function:
+
+```go
+package main
+import (
+	"fmt"
+	"piscine"
+)
+func main() {
+	fmt.Println(piscine.HashCode("A"))
+	fmt.Println(piscine.HashCode("AB"))
+	fmt.Println(piscine.HashCode("BAC"))
+	fmt.Println(piscine.HashCode("Hello World"))
+}
+```
+
+And its output:
+
+```console
+$ go run .
+B
+CD
+EDF
+Spwwz+bz}wo
+```