add new subject for the map exercice in the piscine

subjects/maxwordcountn/README.md

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+## maxwordcountn
+
+### Instructions
+
+Write a function `MaxWordCountN` that will return a `map` of the `n` words that occurs the most in a string `text`. This map will have the word as key and the number of occurences of this word as value.
+If two words have the same number of occurences, the one with the lowest ASCII value should be prioritized.
+
+### Expected function
+
+```go
+func MaxWordCountN(text string, n int) map[string]int {
+
+}
+```
+
+### Usage
+
+Here is a possible program to test your function :
+
+```go
+package main
+
+import (
+	"fmt"
+	"piscine"
+)
+
+func main() {
+	fmt.Println(MaxWordCountN(`
+	Orange Orange is the sun sliding to the horizon after a summer day. Orange is the sound of dribbling basetball. Orange is the smell of a tiger lily petal. Orange is the taste of thirst-quenching Nehi Soda. Orange is the color of peach marmalade on a side of toast. Orange is the sound of a carrot popping out of the ground.
+	`, 3))
+}
+```
+
+And its output :
+
+```console
+$ go run .
+map[Orange:6 is:6 of:7 the:8]
+$
+```