add(subject): rectperimeter exercise

subjects/rectperimeter/README.md

@@ -0,0 +1,32 @@
+# rectperimeter
+
+### Instructions
+
+write a function that accepts two integer arguments, representing the two sides length of a rectangle and returning the perimeter of the rectangle.
+
+your function should return `-1` if one of the arguments is negative.
+
+### Expected function
+
+```go
+func RectPerimeter(l, w int) int {
+
+}
+```
+
+### Usage
+
+
+```go
+package main
+
+import (
+	"fmt"
+	"piscine"
+)
+
+func main() {
+	fmt.Println(piscine.RectPerimeter(10, 2))
+	fmt.Println(piscine.RectPerimeter(434343, 898989))
+}
+```