docs: Add readme for popint subject

subjects/popint/README.md

@@ -0,0 +1,39 @@
+## popint
+
+### Instructions
+
+Write a function that receives a slice of int and returns a new slice without the last element.
+
+### Expected function
+
+```go
+func PopInt(ints []int) []int {
+
+}
+```
+
+### Usage
+
+Here is a possible program to test your function:
+
+```go
+package main
+
+import (
+    "piscine"
+    "fmt"
+)
+
+func main() {
+    ints := {6,7,8,9,}
+    l := piscine.PopInt(ints)
+    fmt.Println(l)
+}
+```
+
+And its output:
+
+```console
+$ go run .
+[6 7 8]
+```