docs(printif): add subject

subjects/printif/README.md

@@ -0,0 +1,46 @@
+## printif
+
+### Instructions
+
+Write a function that takes a `string` as an argument and returns the letter `G` followed by a newline `\n` if the argument length is more than 3, otherwise returns `Invalid Input` followed by a newline `\n`.
+
+- If it's an empty string return `G` followed by a newline `\n`.
+
+### Expected function
+
+```go
+func PrintIfNot(str string) string {
+
+}
+```
+
+### Usage
+
+Here is a possible program to test your function:
+
+```go
+package main
+
+import (
+	"fmt"
+
+	"piscine"
+)
+
+func main() {
+	fmt.Print(piscine.PrintIf("abcdefz"))
+	fmt.Print(piscine.PrintIf("abc"))
+	fmt.Print(piscine.PrintIf(""))
+	fmt.Print(piscine.PrintIf("14"))
+}
+```
+
+And its output:
+
+```console
+$ go run . | cat -e
+G$
+Invalid Output$
+G$
+Invalid Output$
+```