feat| create readme

Author: nik-rev

13_factorial/README.md

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+# Exercise 13 - Factorial
+
+Create a function that takes an input integer greater than or equal to 1 and returns the product of all its predecessing integers. If 0 is input it returns 1. It should accept strings that when converted to a number are valid inputs. For invalid inputs it returns `undefined`.
+
+For example:
+
+```javascript
+factorial(2); // 2 * 1, Output: 2
+factorial('4'); // 4 * 3 * 2 * 1, Output: 24
+factorial(5); // 5 * 4 * 3 * 2 * 1, Output: 120
+factorial(7.2); // Output: undefined
+factorial(0); // Output: 1
+```