feat: subject strisnegative

subjects/strisnegative/README.md

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+## isnegative
+
+Write a function named `IsNegative()` 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 is zero print `0`.
+-  If it's not a number print newline.
+-  Your program should always print `('\n') `at the end of the output.
+
+### Expected function
+```go
+func StrisNegative(str string) {
+
+}
+```
+
+### Usage
+
+Here is a possible program to test your function :
+
+```go
+package main
+
+import "piscine"
+
+func main() {
+	piscine.StrisNegative("585")
+	piscine.StrisNegative("-58")
+	piscine.StrisNegative("55s44")
+}
+```
+
+And its output :
+
+```console
+$ go run .
+P
+N
+!
+```
+