Add missing Javadoc.

Author: Gary Gregory

src/main/java/org/apache/commons/io/input/AbstractCharacterFilterReader.java

@@ -28,6 +28,11 @@
  */
 public abstract class AbstractCharacterFilterReader extends FilterReader {
 
+    /**
+     * Skips nothing.
+     *
+     * @since 2.9.0
+     */
     protected static final IntPredicate SKIP_NONE = ch -> false;
 
     private final IntPredicate skip;