feat(piscine-go): add README.md for new go exercise shoppingsummarycounter

subjects/shoppingsummarycounter/README.md

@@ -0,0 +1,43 @@
+## shoppingsummarycounter
+
+### Instructions
+
+You have a receipt from the grocery store and you want to know how many of each item you bought. Write a function that returns this summary.
+
+Given a `string` count the total amount of times each item appears in it and return a summary including the item and its number of appearances as an `int`.
+
+### Expected function
+
+```go
+func ShoppingSummaryCounter(str string) map[string]int {
+
+}
+```
+
+### Usage
+
+Here is a possible program to test your function:
+
+```go
+package main
+
+import "fmt"
+
+func main() {
+    summary := "Burger Water Carrot Coffee Water Water Chips Carrot Carrot Burger Carrot Water"
+    for index,element := range ShoppingSummaryCounter(summary){
+        fmt.Println(index,"=>",element)
+    }
+}
+```
+
+And its output:
+
+```console
+$ go run . | cat -e
+Burger => 2$
+Water => 4$
+Carrot => 4$
+Coffee => 1$
+Chips => 1$
+```