docs(splitStringIntoString): Adding the subject of exercice spliStringIntoPairs for js piscine

subjects/splitStringIntoPairs/README.md

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+## splitStringIntoPairs
+
+### Instructions
+
+Splits the string into pairs of two characters. 
+If the string contains an odd number of characters then it should replace the missing second character of the final pair with an underscore ('_')
+### Expected function
+ 
+
+```js
+function  splitStringIntoPairs(str) {
+}
+```
+
+### Usage
+
+Here is a possible program to test your function :
+
+```js
+console.log(splitStringIntoPairs("abc"));
+console.log( splitStringIntoPairs("abcdef"));
+```
+
+And its output :
+
+```console
+$ node index.js
+$ ['ab', 'c_']
+$ ['ab', 'cd', 'ef']
+
+```