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

subjects/suffix/README.md

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+## suffix
+
+### Instructions
+
+Create function to move the first letter of each word to the end and add "er" to the end of each word while leaving punctuation marks untouched using regular expression
+
+### Expected function
+
+```js
+function suffix(str) {
+}
+```
+
+### Usage
+
+Here is a possible program to test your function:
+
+```js
+const inputStr = "  this is a  test string   ";
+const format = suffix(inputStr);
+console.log(format);
+```
+
+and the output should be:
+
+```console
+$ node index.js
+"elloHer, orldwer! hisTwer is Her a estwer."
+$
+```