docs: adding subject

subjects/java/checkpoints/factorial/README.md

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+## Factorial
+
+### Instructions
+
+In a file named `Factorial.java` write a function `factorial` that calculates the factorial of a non-negative integer given as a parameter.
+
+> The factorial of a non-negative integer n (denoted as n!) is the product of all positive integers less than or equal to n. For example, 5! = 5 × 4 × 3 × 2 × 1 = 120.
+
+### Expected Functions
+
+```java
+public class Factorial {
+    public static Integer factorial(Integer n) {
+        // your code here
+    }
+}
+```
+
+### Usage
+
+Here is a possible `ExerciseRunner.java` to test your function :
+
+```java
+public class ExerciseRunner {
+    public static void main(String[] args) {
+        System.out.println(Factorial.factorial(3));
+    }
+}
+```
+
+and its output :
+
+```shell
+$ javac *.java -d build
+$ java -cp build ExerciseRunner
+6
+$
+```