added comment

Author: HunorVadaszPerhat

inner_class/static_inner_class/src/Outer.java

@@ -2,6 +2,7 @@ public class Outer {
     static int x = 10;
     int y = 20;
 
+    // You cannot use the static keyword with a class unless it is an inner class
     static class Inner {
         void display(){
             System.out.println("From Outer class, x is: " + x);