docs(vowelSing): Adding the subject of exercice vowelSign for js piscine

subjects/vowelSign/README.md

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+## vowelSign
+
+### Instructions
+
+Create function named "VowelSign" that takes a string as its input parameter.
+
+The function is expected to produce a modified string as its output, where in each uppercase vowel in the input string is replaced with a "+" symbol, and each lowercase vowel is substituted with a "-" symbol.
+
+It is required to implement this functionality by defining a regular expression pattern for identifying vowels, which should be stored in a variable named "vowels."
+
+### Expected function
+
+```js
+function VowelSign(inputString) {
+}
+
+```
+### Usage
+
+Here is a possible program to test your function
+
+```js
+
+const inputStr = "This Is a Sample StrIng";
+const result = VowelSign(inputStr);
+console.log(result);
+
+```
+And its output :
+
+```console
+$ node index.js
+"Th-s +s a S-mpl- Str+ng
+
+```