docs(numberOfLetters): Adding the subject of exercice numbersOfLetters for js piscine

subjects/numberOfLetters/README.md

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+## numberOfLetters
+
+### Instructions
+
+Create a function that if we write out the digits of "60" as English words we get "sixzero"; the number of letters in "sixzero" is seven.
+
+The number of letters in "seven" is five. The number of letters in "five" is four. The number of letters in "four" is four: we have reached a stable equilibrium.
+
+Note: for integers larger than 9, write out the names of each digit in a single word (instead of the proper name of the number in English). 
+
+For example, write 12 as "onetwo" (instead of twelve), and 999 as "nineninenine" (instead of nine hundred and ninety-nine).
+
+For any integer between 0 and 999, return an array showing the path from that integer to a stable equilibrium.
+
+### Expected function
+
+```js
+function numberOfLetters(int) {
+}
+```
+
+### Usage
+
+Here is a possible program to test your function:
+
+```js
+console.log(numbersOfLetters(60))
+console.log(numbersOfLetters(1))
+```
+
+and the output should be:
+
+```console
+$ node index.js
+["sixzero", "seven", "five", "four"]
+["one", "three", "five", "four"]
+$
+```
+
+
+