update: add usage

Sort.java

@@ -29,6 +29,12 @@ public class Sort {
   }
 
   public static void main(String[] args) {
+    if (args.length == 0) {
+      System.err.println("Usage: <number>...");
+      System.exit(1);
+      return;
+    }
+
     int[] input = Arrays.stream(args).mapToInt(Integer::parseInt).toArray();
     System.out.println("Input:  " + Arrays.toString(input));
     System.out.println("Output: " + Arrays.toString(selectionSort1(input)));