modify README.md

Author: hujunxianligong

src/main/java/cn/edu/hfut/dmic/webcollector/example/DemoNextFilter.java

@@ -77,6 +77,7 @@ public CrawlDatum filter(CrawlDatum nextItem, CrawlDatum referer) {
                     nextItem.type("content");
                     return nextItem;
                 } else {
+                    // return null if you want to filter the CrawlDatum
                     return null;
                 }
             }