docs: adding subject

subjects/java/checkpoints/strlen/README.md

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+## 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
+$
+```