CON-2541-Markdown-Strlen-exercise (#2523)

* docs: adding subject

* docs: rephrase exercice description to improve its clarity

---------

Co-authored-by: amine <aaitihia@leyton.com>

subjects/java/checkpoints/strlen/README.md

@@ -0,0 +1,36 @@
+## Strlen
+
+### Instructions
+
+In a file named `Strlen.java` write a function `strlen` that returns the length of the string passed as a parameter.
+
+### Expected Functions
+
+```java
+public class Strlen {
+    public static int strlen(String s) {
+        // 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(Strlen.strlen("Hello World !"));
+    }
+}
+```
+
+and its output :
+
+```shell
+$ javac *.java -d build
+$ java -cp build ExerciseRunner
+13
+$
+```