docs: adding subject

subjects/java/checkpoints/palindrom/README.md

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+## Palindrome
+
+### Instructions
+
+In a file named `Palindrome.java` write a function `isPalindrome` that returns true if the String in a parameter is a palindrome, i.e. Have the same spelling backward and forwards (e.g. 'kayak').
+
+### Expected Functions
+
+```java
+public class Palindrome {
+    public static boolean isPalindrome(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(Palindrome.isPalindrome("ressasser"));
+        System.out.println(Palindrome.isPalindrome("Bonjour"));
+    }
+}
+```
+
+and its output :
+
+```shell
+$ javac *.java -d build
+$ java -cp build ExerciseRunner
+true
+false
+$
+```