subject(Add-if-positive):add readme

subjects/addifpositive/README.md

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+## add-if-positive 
+
+### Instructions
+Write a function that takes two numbers and adds them together if they are both positive.
+- If either number is negative or one of them is negative and the other is positive return `0`.
+
+### Expected function 
+```go
+func AddIfPositive(a int, b int) int {
+    // your code here
+}
+```
+
+### Usage
+
+```go
+package main
+
+import "fmt"
+
+func main() {
+    fmt.Println(AddIfPositive(1, 2))
+    fmt.Println(AddIfPositive(1, -2))
+    fmt.Println(AddIfPositive(-1, 2))
+    fmt.Println(AddIfPositive(-1, -2))
+    fmt.Println(AddIfPositive(10,20))
+}
+```
+
+and the output should be:
+
+```console
+$ go run .
+3
+0
+0
+0
+30
+```