Factorial README file added

Author: me-shaon

Number Theory/Factorial/README.md

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+# Factorial
+
+In mathematics, the **factorial** of a non-negative integer n, denoted by **n!**, is the product of all positive integers less than or equal to n. For example:
+
+```
+ 5! = 1 * 2 * 3 * 4 * 5
+    = 120
+```
+
+We can easily calculate a factorial from the previous one:
+
+![Factorial Chart](./images/factorial_chart.png)
+
+So, the rule is:
+```
+n! = n × (n−1)!
+```
+> The value of 0! is 1, according to the convention for an [empty product](https://en.wikipedia.org/wiki/Empty_product).
+
+Factorials are used in many areas of mathematics, but particularly in Combinations and Permutations.
+
+### More on this topic
+- [Wikipedia](https://en.wikipedia.org/wiki/Factorial)
+- [Factorial - MathisFun](https://www.mathsisfun.com/numbers/factorial.html)
+- [khanacademy](https://www.khanacademy.org/computing/computer-science/algorithms/recursive-algorithms/a/the-factorial-function)