docs: adding subject (#2524)

subjects/java/checkpoints/count-char/README.md

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+## CountChar
+
+### Instructions
+
+In a file named `CountChar.java` write a function `count` that takes two parameters a string and a char, then return the count of that char in the given string.
+
+### Expected Functions
+
+```java
+public class CountChar {
+    public static int count(String s, char c) {
+        // 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(CountChar.count("Hello World !", ' '));
+    }
+}
+```
+
+and its output :
+
+```shell
+$ javac *.java -d build
+$ java -cp build ExerciseRunner
+2
+$
+```