docs: adding subject

subjects/java/checkpoints/average-calc/README.md

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+## AverageCalc
+
+### Instructions
+
+Write a function named `average` in a file named `AverageCalc` java. This function takes three parameters: start, end, and step. It returns the average of the numbers between start and end with each number being greater than the previous one by the step given."
+
+### Expected Functions
+
+```java
+public class AverageCalc {
+    public static int average(int start, int end, int step) {
+        // 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(MultiplicationTable.generate(1,5,1));
+    }
+}
+```
+
+and its output :
+
+```shell
+$ javac *.java -d build
+$ java -cp build ExerciseRunner
+3
+$
+```