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

subjects/asterisksfirst/README.md

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+## asterisksfirst
+
+### Instructions
+
+Create function that processes a string.
+
+It should trim spaces from the beginning and end of the string and capitalize the first letter of each word. 
+
+Furthermore, it should remove spaces between words and prepend the string with an asterisk (*).
+
+The function should return the processed string, but if the resulting string exceeds 250 characters in length, or if the input or output is empty, it should return 'false'.
+
+### Expected function
+
+```js
+function asterisksfirst(inputString) {
+}
+```
+
+### Usage
+
+Here is a possible program to test your function:
+
+```js
+const inputString = "  this is a  test string   ";
+const format = asterisksfirst(inputString);
+console.log(format);
+```
+
+and the output should be:
+
+```console
+$ node index.js
+"*ThisIsATestString"
+$
+```