CON-#### Add IS-Negative subject

subjects/str_isnegative/README.md

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+
+# IS-Negative
+
+### Instructions
+
+Write a function named `StrisNegative()` that defines if a number (You should check if it's a number) is negative or positive.
+
+- Your function print P if the number is positive
+- Your function print F if the number is negative
+- For the number 0 is zero print 0.
+- Any input other than number should print !
+- Your function should always print ('\n')at the end of the output.
+
+### Expected function
+
+```go
+func StrisNegative(str string){
+    //code
+}
+```
+
+### Usage
+
+```go
+package main
+
+import (
+	"os"
+    "piscine"
+	"fmt"
+)
+
+func main() {
+	argument := os.Args[1:]
+	piscine.StrisNegative(argument[0])
+}
+```
+And its output:
+
+```console
+$ go run . | cat -e
+!$
+$ go run . "7" | cat -e
+P$
+$ go run . "-9" | cat -e
+N$
+$ go run . a | cat -e
+!$
+$ go run . "-552" | cat -e
+N$
+$ go run . "58" | cat -e
+P$
+$ go run . "Hello World" | cat -e
+!$
+```