add(subjects): zipstring exercice subject

subjects/zipstring/README.md

@@ -0,0 +1,41 @@
+## zipstring
+
+### Instructions
+
+Write a function that takes a `string` and returns a new string that replaces every character with the number of duplicates and the character itself, deleting the extra duplications.
+
+The letters are from the Latin alphabet list only, any other characters, symbols or empty spaces shall not be tested.
+
+### Expected function
+
+```go
+func ZipString(s string) string {
+
+}
+```
+
+### Usage
+
+Here is a possible program to test your function :
+
+```go
+package main
+
+import (
+	"fmt"
+	"piscine"
+)
+
+func main() {
+	fmt.Println(piscine.ZipString("YouuungFellllas"))
+}
+
+```
+
+And its output :
+
+```console
+$ go run .
+1Y1o3u1n1g1F1e4l1a1s
+$
+```